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11 Historical Uses for Invisible Ink

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The history of invisible ink veers wildly back and forth between high-tech methods and the humblest of approaches. In her book, Prisoners, Lovers, and Spies: The Story of Invisible Ink from Herodotus to al-Qaeda, Kristie Macrakis traces invisible ink from daring escapes to love affairs to acts of espionage.

1. Ovid's Advice

The Roman poet Ovid, a noted ladies’ man, wrote elaborate instructions to lovers in his Ars Amatoria (Art of Love). In the section directed at women, Ovid taught wives bent upon deceiving their husbands to communicate in secret. Ask a servant “who is in your secrets” to carry missives in her stocking or her bosom, “under a wide shawl.” Or, failing that, try secret writing, which was apparently common knowledge by 18 BCE, when the book was written. “Characters written in fresh milk are a well-known means of secret communication,” the poet wrote. “Touch them with a little powdered charcoal and you will read them.”

2. Mary's Downfall

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The Catholic Mary, Queen of Scots, kept under luxurious house arrest for eighteen years by her Protestant cousin Elizabeth I, used invisible ink and cipher to communicate with Catholic supporters on the outside. Mary advised correspondents to write to her employing two commonly used substances: alum (hydrated potassium aluminum sulfate) or nutgall (the tannic acid secreted in swellings generated by parasitic wasps colonizing oak trees). Letters written in alum required the recipient to soak the paper in water, while nutgall needed a solution of ferrous sulphate as a developer. Eventually, Lord Walsingham, Elizabeth I’s spymaster, who had been breaking Mary’s codes all along, set a trap for Mary, using a double agent to entice her into partially committing to a plot against Elizabeth’s life. Mary was executed on February 8, 1587.

3. Orange You Glad...

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Another Catholic prisoner in England used invisible ink with far happier results in 1597. The Jesuit priest John Gerard came to England in 1588 to carry out a secret mission for the Catholic underground. Caught and detained in the Tower of London, Gerard was tortured for information. The priest befriended his prison guard and began to ask for oranges, whose juice he saved to write to confederates to on the outside. With the help of this guard, Gerard even communicated with a fellow Catholic prisoner whose cell he could see from his own, miming directions for developing the orange-juice letters over flame. The two eventually conspired to escape the Tower, with the help of outside accomplices who brought a rope—a feat made more impressive by the fact that Gerard’s fingers had been wrecked during his torture sessions.

4. Explosive Ink

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During the eighteenth century, the new vogue for popular science in France and England made invisible ink into entertainment, enacted in public in front of wondering eyes. Poor Jean-Jacques Rousseau experimented with a dangerous type of sympathetic ink in 1736. Macrakis writes that Rousseau probably heard of the recipe from a professor friend, or read about it in a book of recreational experiments. The ink was made with quicklime and orpiment (a rare mineral that’s pigmented orange or yellow, and contains arsenic sulfide). When the philosopher mixed the two, the bottle began to fizz uncontrollably, and eventually exploded in his face. “He swallowed so much chalk and orpiment that it nearly killed him,” Macrakis writes. “He couldn’t see for more than six weeks.”

5. Washington's "Medicine"

The Culper Spy Ring were agents for George Washington who circulated in occupied New York City from 1778 to 1783. The group, recruited by Major Benjamin Tallmadge, used pseudonyms and numerical codes to pass information, fearful as they were of being discovered. They also used invisible ink, manufactured for Washington by James Jay, a doctor who was John Jay’s older brother. This combination of precautions meant that they all got through the war without being discovered, and managed to feed Washington some valuable bits of strategic information. Jay didn’t record the chemical makeup of the fluid, which he and Washington called “the medicine” in letters to each other. In the 1930s, curious doctor and photographer Dr. Lodewyk Bendikson performed ultraviolet and infrared tests on letters written using Jay’s invisible ink. Bendikson found that Jay’s formula was an old one: tannic acid from gallnuts, developed with ferrous sulfate.

6. When Life Hands You Lemons

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In World War I, several so-called “lemon juice spies”—German agents operating in England—used citrus as their means of communication. The British government had stepped up its censorship of letters in wartime. One agent, Mabel Beatrice Elliot, flagged letters written by three of these men, heated them up, and unmasked them as spies. The lemon-juice operation was a clumsy one: several spies, once caught, had lemons on their persons, or pens with pulp still stuck on the nibs. In the end, the British executed 11 German spies in the Tower of London in 1915; four of them had used lemon juice. “After the painful and visible loss of … the lemon juice spies,” Macrakis writes, “the Germans began to develop more sophisticated invisible ink methods.”

7. Bacon's Blunder

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George Vaux Bacon, an American journalist recruited by Germans to spy in Britain, was the beneficiary of these new ideas. He smuggled a new kind of invisible ink into the country in a novel way. Bacon’s handler asked him to buy black socks, and then impregnated the socks with the paste-like ink, telling him to soak the socks in water once he reached his destination. Bacon was quickly nabbed, after censors suspected his letters based on their address in the Netherlands. The substance on his socks was Argyrol, a drug used as an antiseptic and anti-bacterial that contains a mild silver protein. Bacon didn’t know how to develop the ink, which was totally new to British censors; eventually, British and French chemists figured out a method using electrolysis. A British court condemned Bacon to death, but his sentence was commuted in exchange for his testimony against his German handlers. Later, he claimed that the whole episode had been a stunt, meant to result in a great magazine story about spying.

8. Flu Ink

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During World War II, chemist Linus Pauling worked on an unusual wartime project, formulating new kinds of invisible ink that would resist all known reagents. Pauling and his colleagues experimented with invisible inks made from pneumococcus bacteria—inert, in this preparation, and so unable to spread pneumonia. The ink-ified microbe would react to an antibody, and then become visible once dipped in a dye solution. The ink never passed the experimental stage; neither did the various inks made with radioactive isotopes that were tested by MIT physicist Robley Evans.

9. What is This? Why is it Here?

James Stockdale, a Navy pilot, was shot down over North Vietnam in 1965 and sent to the “Hanoi Hilton,” where he was to stay for seven and a half years. With the help of U.S. Naval Intelligence, his wife Sybil initiated secret communications with Stockdale by enclosing a photograph of her mother in a letter to him. He was confused, but (as he said later) he thought: “It’s dumb to throw away something from the States without doing more with it. James Bond would soak it in piss and see if a message came out of it.” So he did. After it dried, print appeared on the back, establishing the code that he later used to communicate with the Navy, informing them of conditions at the prison.

10. Ultraviolet Exposure

The East German secret police—the Stasi—prescreened 90,000 pieces of mail every day during the 1980s, using an elaborate conveyor belt system. Agents steamed open letters in bulk, identified suspicious pieces with indentations or scratch marks, then glued them shut, in assembly-line fashion. Renate Murk, a Stasi captain who examined the letters that were under suspicion, used new technology also favored by the CIA to reveal secret writing without using a reagent. (If you chemically develop a letter, that’s an irreversible process, and you can’t send the letter on to its intended recipient; any element of surprise is lost.) Murk used ultraviolet scanners and the Nyom invisible impressions detector to find invisible writing without developing the letter.

11. Prison Speak

In the 1990s, the Aryan Brotherhood used citrus juice and urine to send messages between prisons, orchestrating violent action. In 1997, Brotherhood leader T.D. Bingham, imprisoned at the Supermax prison in Fremont County, Colorado, sent a letter to an outside courier, who passed it to Brotherhood members imprisoned in Lewisburg, Penn. The letter was written in urine, and revealed its secrets upon being “toasted” over a flame. The message: “War with DC Blacks, T.D.” When Bingham and other Brotherhood leaders were tried for ordering this and other attacks in 2006, embarrassed government censors had to admit that they’d missed the message completely.

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The History Behind Why We Eat 10 Dishes at Thanksgiving
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Halloween is for candy comas, and on Independence Day we grill, but no holiday is as completely defined by its cuisine as Thanksgiving. No matter what part of the country you're in, it's a safe bet that at least a few of the below dishes will be making an appearance on your table this week. But what makes these specific entrees and side dishes so emblematic of Thanksgiving? Read on to discover the sometimes-surprising history behind your favorite fall comfort foods.

1. TURKEY

A roasted turkey on a platter.
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Turkey has become so synonymous with Thanksgiving that most of us probably imagine the pilgrims and Wampanoag tribe of Native Americans chowing down on a roast bird in 1621. Although we don't know the exact menu of that first Plymouth Colony feast, a first-person account of the year's harvest from governor William Bradford does reference "a great store of wild turkeys," and another first-person account, from colonist Edward Winslow, confirms that the settlers "killed as much fowl as…served the company almost a week." However, culinary historian Kathleen Wall believes that, although turkeys were available, it's likely that duck, goose, or even passenger pigeons were the more prominent poultry options at the first Thanksgiving. Given their proximity to the Atlantic, local seafood like oysters and lobsters were likely on the menu as well.

As the holiday grew in popularity, however, turkey became the main course for reasons more practical than symbolic. English settlers were accustomed to eating fowl on holidays, but for early Americans, chickens were more valued for their eggs than their meat, and rooster was tough and unappetizing. Meanwhile, turkeys were easy to keep, big enough to feed a whole family, and cheaper than ducks or geese. Even before Thanksgiving was recognized as a national holiday, Alexander Hamilton himself remarked that "No citizen of the U.S. shall refrain from turkey on Thanksgiving Day." The country followed his advice: according to the National Turkey Federation, 88 percent of Americans will eat turkey in some form on Thanksgiving Day—an estimated 44 million birds!

2. STUFFING

Pan of breaded stuffing.
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Stuffing would have been a familiar concept to those early settlers as well, although their version was likely quite different from what we're used to. We know that the first Plymouth colonists didn't have access to white flour or butter, so traditional bread stuffing wouldn't have been possible yet. Instead, according to Wall, they may have used chestnuts, herbs, and chunks of onion to flavor the birds, all of which were already part of the local fare. Centuries later, we're still stuffing turkeys as a way to keep the bird moist through the roasting process and add extra flavor.

3. CRANBERRIES

Dish of cranberry sauce.
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Like turkeys, cranberries were widely available in the area, but cranberry sauce almost certainly did not make an appearance at the first Thanksgiving. Why not? The sugar reserves the colonists would have had were almost completely depleted after their long sea journey, and thus they didn't have the means to sweeten the terrifically tart berries.

So how did cranberries become such an autumnal staple? For starters, they're a truly American food, as one of only a few fruits—along with Concord grapes, blueberries, and pawpaws—that originated in North America. They grow in such abundance in the northeast that colonists quickly began incorporating cranberries into various dishes, such as pemmican, which mixed mashed cranberries with lard and dried venison. By the Civil War, they were such a holiday staple that General Ulysses S. Grant famously demanded his soldiers be provided cranberries for their Thanksgiving Day meal.

4. MASHED POTATOES

Bowl of mashed potatoes.
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Potatoes weren't yet available in 17th-century Plymouth, so how did mashed potatoes become another Thanksgiving superstar? The answer lies in the history of the holiday itself. In America’s earliest years, it was common for the sitting President to declare a "national day of thanks," but these were sporadic and irregular. In 1817, New York became the first state to officially adopt the holiday, and others soon followed suit, but Thanksgiving wasn't a national day of celebration until Abraham Lincoln declared it so in 1863.

Why did Lincoln—hands full with an ongoing war—take up the cause? Largely due to a 36-year campaign from Sarah Josepha Hale, a prolific novelist, poet, and editor, who saw in Thanksgiving a moral benefit for families and communities. In addition to her frequent appeals to officials and presidents, Hale wrote compellingly about the holiday in her 1827 novel Northwood, as well as in the womens' magazine she edited, Godey's Lady's Book. Her writing included recipes and descriptions of idealized Thanksgiving meals, which often featured—you guessed it—mashed potatoes.

5. GRAVY

Plate of turkey and potatoes covered in gravy.
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Despite a dearth of potatoes, it's likely that some type of gravy accompanied the turkey or venison at the earliest Thanksgiving gatherings. The concept of cooking meat in sauce dates back hundreds of years, and the word "gravy" itself can be found in a cookbook from 1390. Because that first celebration extended over three days, historian Wall speculates: "I have no doubt whatsoever that birds that are roasted one day, the remains of them are all thrown in a pot and boiled up to make broth the next day." That broth would then be thickened with grains to created a gravy to liven day-old meat. And, if Wall's correct, that broth sounds suspiciously like the beginning of another great Thanksgiving tradition: leftovers!

6. CORN

Plate of corn.
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Corn is a natural symbol of harvest season—even if you're not serving it as a side dish, you might have a few colorful ears as a table centerpiece. We know that corn was a staple of the Native American diet and would have been nearly as plentiful in the 17th century as today. But according to the History Channel, their version would have been prepared quite differently: corn was either made into a cornmeal bread or mashed and boiled into a thick porridge-like consistency, and perhaps sweetened with molasses. Today, we eat corn in part to remember those Wampanoag hosts, who famously taught the newcomers how to cultivate crops in the unfamiliar American soil.

7. SWEET POTATOES

Bowl of mashed sweet potatoes.
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In the midst of so many New England traditions, the sweet potatoes on your table represent a dash of African-American culture. The tasty taters originally became popular in the south—while pumpkins grew well in the north, sweet potatoes (and the pies they could make) became a standard in southern homes and with enslaved plantation workers, who used them as a substitution for the yams they'd loved in their homeland. Sweet potato pie was also lovingly described in Hale's various Thanksgiving epistles, solidifying the regional favorite as a holiday go-to. More recently, some families further sweeten the dish by adding toasted marshmallows, a love-it-or-hate-it suggestion that dates to a 1917 recipe booklet published by the Cracker Jack company.

8. GREEN BEAN CASSEROLE

Plate of green bean casserole.
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Beans have been cultivated since ancient times, but green bean casserole is a decidedly modern contribution to the classic Thanksgiving canon. The recipe you love was whipped up in 1955 by Dorcas Reilly, a home economist working in the Campbell's Soup Company test kitchens in Camden, New Jersey. Reilly's job was to create limited-ingredient recipes that housewives could quickly replicate (using Campbell's products, of course). Her original recipe (still available at Campbells.com), contains just six ingredients: Campbell's Cream of Mushroom soup, green beans, milk, soy sauce, pepper, and French's French Fried Onions. Her recipe was featured in a 1955 Associated Press feature about Thanksgiving, and the association has proven surprisingly durable—Campbell’s now estimates that 30 percent of their Cream of Mushroom soup is bought specifically for use in a green bean casserole.

9. PUMPKIN PIE

Slice of pumpkin pie.
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Like cranberries, pumpkin pie does have ties to the original Thanksgiving, albeit in a much different format. The colonists certainly knew how to make pie pastry, but couldn't have replicated it without wheat flour, and might have been a bit perplexed by pumpkins, which were bigger than the gourds they knew in Europe. According to Eating in America: A History, however, Native Americans were already using the orange treats as a dessert meal: "Both squash and pumpkin were baked, usually by being placed whole in the ashes or embers of a dying fire and they were moistened afterwards with some form of animal fat, or maple syrup, or honey." It's likely that Hale was inspired by those stories when pumpkin pie appeared in her culinary descriptions.

10. WINE

Two glasses of wine.
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Chances are good that a few glasses of wine will be clinked around your table this November, but did the pilgrims share a tipsy toast with their new friends? Kathleen Wall thinks that water was probably the beverage of choice, considering that the small amount of wine the settlers had brought with them was likely long gone. Beer was a possibility, but since barley hadn't been cultivated yet, the pilgrims had to make do with a concoction that included pumpkins and parsnips. Considering the availability of apples in what would become Massachusetts, however, other historians think it's possible that hard apple cider was on hand for the revelers to enjoy. Whether or not the original feast was a boozy affair, cider rapidly became the drink of choice for English settlers in the area, along with applejack, apple brandy, and other fruit-based spirits. New England cider thus indirectly led to a less-beloved Thanksgiving tradition: your drunk uncle's annual political rant. Bottoms up!

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The Origins of 5 International Food Staples

Food is more than fuel. Cuisine and culture are so thoroughly intertwined that many people automatically equate tomatoes with Italy and potatoes with Ireland. Yet a thousand years ago those dietary staples were unheard of in Europe. How did they get to be so ubiquitous there—and beyond?

1. TOMATOES

For years, the wonderful fruit that’s now synonymous with Italy was mostly ignored there. Native to South America and likely cultivated in Central America, tomatoes were introduced to Italy by Spanish explorers during the 1500s. Shortly thereafter, widespread misconceptions about the newcomers took root. In part due to their watery complexion, it was inaccurately thought that eating tomatoes could cause severe digestive problems. Before the 18th century, the plants were mainly cultivated for ornamental purposes. Tomato-based sauce recipes wouldn’t start appearing in present-day Italy until 1692 (although even those recipes were more like a salsa or relish than a sauce). Over the next 150 years, tomato products slowly spread throughout the peninsula, thanks in no small part to the agreeable Mediterranean climate. By 1773, some cooks had taken to stuffing tomatoes with rice or veal. In Naples, the fruits were sometimes chopped up and placed onto flatbread—the beginnings of modern pizza. But what turned the humble tomato into a national icon was the canning industry. Within Italy’s borders, this business took off in a big way during the mid-to-late 19th century. Because tomatoes do well stored inside metal containers, canning companies dramatically drove up the demand. The popularity of canned tomatoes was later solidified by immigrants who came to the United States from Italy during the early 20th century: Longing for Mediterranean ingredients, transplanted families created a huge market for Italian-grown tomatoes in the US.

2. CURRY

Bowl of chicken curry with a spoon in it

An international favorite, curry is beloved in both India and the British Isles, not to mention the United States. And it turns out humans may have been enjoying the stuff for a very, very long time. The word “curry” was coined by European colonists and is something of an umbrella term. In Tamil, a language primarily found in India and Sri Lanka, “kari” means “sauce.” When Europeans started traveling to India, the term was eventually modified into “curry,” which came to designate any number of spicy foods with South or Southeast Asian origins. Nonetheless, a great number of curry dishes share two popular components: turmeric and ginger. In 2012, traces of both were discovered inside residue caked onto pots and human teeth at a 4500-year-old archaeological site in northern India. And where there’s curry, there’s usually garlic: A carbonized clove of this plant was also spotted nearby. “We don’t know they were putting all of them together in a dish, but we know that they were eating them at least individually,” Steve Weber, one of the archaeologists who helped make this astonishing find, told The Columbian. He and his colleagues have tentatively described their discovery as "proto-curry."

3. THE BAGUETTE

Several baguettes

A quintessential Gallic food, baguettes are adored throughout France, where residents gobble up an estimated 10 billion every year. The name of the iconic bread ultimately comes from the Latin word for stick, baculum, and references its long, slender form. How the baguette got that signature shape is a mystery. One popular yarn credits Napoleon Bonaparte: Supposedly, the military leader asked French bakers to devise a new type of skinny bread loaf that could be comfortably tucked into his soldiers’ pockets. Another origin story involves the Paris metro, built in the 19th century by a team of around 3500 workers who were apparently sometimes prone to violence during meal times. It’s been theorized that the metro foremen tried to de-escalate the situation by introducing bread that could be broken into pieces by hand—thereby eliminating the need for laborers to carry knives. Alas, neither story is supported by much in the way of historical evidence. Still, it’s clear that lengthy bread is nothing new in France: Six-foot loaves were a common sight in the mid-1800s. The baguette as we know it today, however, didn’t spring into existence until the early 20th century. The modern loaf is noted for its crispy golden crust and white, puffy center—both traits made possible by the advent of steam-based ovens, which first arrived on France’s culinary scene in the 1920s.

4. POTATOES

Bowl of red, white, and black potatoes on wooden table

Historical records show that potatoes reached Ireland by the year 1600. Nobody knows who first introduced them; the list of potential candidates includes everyone from Sir Walter Raleigh to the Spanish Armada. Regardless, Ireland turned out to be a perfect habitat for the tubers, which hail from the misty slopes of the Andes Mountains in South America. Half a world away, Ireland’s rich soils and rainy climate provided similar conditions—and potatoes thrived there. They also became indispensable. For millennia, the Irish diet had mainly consisted of dairy products, pig meats, and grains, none of which were easy for poor farmers to raise. Potatoes, on the other hand, were inexpensive, easy to grow, required fairly little space, and yielded an abundance of filling carbs. Soon enough, the average Irish peasant was subsisting almost entirely on potatoes, and the magical plant is credited with almost single-handedly triggering an Irish population boom. In 1590, only around 1 million people lived on the island; by 1840, that number had skyrocketed to 8.2 million. Unfortunately, this near-total reliance on potatoes would have dire consequences for the Irish people. In 1845, a disease caused by fungus-like organisms killed off somewhere between one-third and one-half of the country’s potatoes. Roughly a million people died as a result, and almost twice as many left Ireland in a desperate mass exodus. Yet potatoes remained a cornerstone of the Irish diet after the famine ended; in 1899, one magazine reported that citizens were eating an average of four pounds’ worth of them every day. Expatriates also brought their love of potatoes with them to other countries, including the U.S. But by then, the Yanks had already developed a taste for the crop: The oldest record of a permanent potato patch on American soil dates back to 1719. That year, a group of farmers—most likely Scots-Irish immigrants—planted one in the vicinity of modern-day Derry, New Hampshire. From these humble origins, the potato steadily rose in popularity, and by 1796, American cookbooks were praising its “universal use, profit, and easy acquirement.”

5. CORN

Corn growing in a field

In the 1930s, geneticist George W. Beadle exposed a vital clue about how corn—also known as maize—came into existence. A future Nobel Prize winner, Beadle demonstrated that the chromosomes found in everyday corn bear a striking resemblance to those of a Mexican grass called teosinte. At first glance, teosinte may not look very corn-like. Although it does have kernels, these are few in number and encased in tough shells that can easily chip a human tooth. Nonetheless, years of work allowed Beadle to prove beyond a shadow of a doubt that corn was descended from teosinte. Today, genetic and archaeological data suggests that humans began the slow process of converting this grass into corn around 8700 years ago in southwestern Mexico. If you're wondering why early farmers showed any interest in cultivating teosinte to begin with, while the plant is fairly unappetizing in its natural state, it does have a few key attributes. One of these is the ability to produce popcorn: If held over an open fire, the kernels will “pop” just as our favorite movie theater treat does today. It might have been this very quality that inspired ancient horticulturalists to tinker around with teosinte—and eventually turn it into corn

BONUS: TEA

Person sitting cross-legged holding a cup of green tea

The United Kingdom’s ongoing love affair with this hot drink began somewhat recently. Tea—which is probably of Chinese origin—didn’t appear in Britain until the 1600s. Initially, the beverage was seen as an exotic curiosity with possible health benefits. Shipping costs and tariffs put a hefty price tag on tea, rendering it quite inaccessible to the lower classes. Even within England’s most affluent circles, tea didn’t really catch on until King Charles II married Princess Catherine of Braganza. By the time they tied the knot in 1662, tea-drinking was an established pastime among the elite in her native Portugal. Once Catherine was crowned Queen, tea became all the rage in her husband’s royal court. From there, its popularity slowly grew over several centuries and eventually transcended socioeconomic class. At present, the average Brit drinks an estimated three and a half cups of tea every day.

All photos courtesy of iStock.

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