The Halloween Science FAQ

What is dry ice and how does it make that awesome fog?

Dry ice is the colorless, odorless, solid form of carbon dioxide, first reported in 1834 by the French chemist Charles Thilorier, who opened a container of liquid carbon dioxide needed for an experiment and observed that most of the liquid CO2 quickly evaporated, leaving a solid form on the bottom of the canister.

The surface temperature of dry ice is −109.3 °F. As it warms up, it sublimes, or transitions from the solid to gas form with no intermediate liquid form (a process called sublimation). These two characteristics make it an excellent coolant and since 1925, when solid CO2 was trademarked and sold as "Dry ice" by the DryIce Corporation of America, it's been used to flash freeze and refrigerate food and biological samples, make ice cream, bait mosquito traps (they're attracted to CO2) and make fog for theater productions, Sunn O))) concerts and haunted houses.

That fog is made by quickly changing the CO2 into its gas form. In an ice chest, dry ice sublimes at an average rate of 5-10 pounds every 24 hours. But placing dry ice in hot water accelerates sublimation considerably and turns the solid CO2 into CO2 gas. The cold CO2 gas meets the surrounding air and drops its temperature enough for condensation to occur and tiny droplets of water to form in the air and, voila, you have fog. Because carbon dioxide is heavier than air, and cold air is denser than warm air, the fog stays low to the ground for that extra creepy effect.

Why do we get goosebumps?

Goose bumps, also called goose flesh or goose pimples and known to medical professionals as cutis anserina ("cutis," skin + "anser," goose = goose skin) involuntarily develop on our skin when we become cold or experience strong emotions in a reflex called horripilation or piloerection. Whether we're freezing or getting the bejesus scared out of us, our sympathetic nervous systems pick up on a fight-or-flight situation and release adrenaline, muscles at the base of our body hairs contract, pull the hair erect, and create a shallow depression on the skin surface that causes the surrounding area to protrude. A goose bump is born.

In mammals with plenty of body hair or fur (chimps, otters, mice, cats, etc.), horripilation serves two purposes. One, erect hairs trap air, create insulation and aid heat retention. Two, erect hairs make an animal appear larger and helps intimidate enemies. In humans, horripilation as a response to cold or fear provides no known benefit since we lost most of our body hair some time ago.

What's the best candy container for trick-or-treating?

hwcandy_03What sort of container will provide you with maximum space for your candy haul? A bucket? A bag? The ol' pillow case? The guys (Guys? Gals? Robots? Not a whole lot of info available on who runs it.) at My Science Project conducted an experiment to find out.

First, the researchers accounted for the wide variety of candies available to the average trick-or-treater. They divided candy into three categories: ""˜premium' (fun-sized candy bars), "˜meh' (chewy boxed candies like Milk Duds), and "˜bottom of the barrel' (hard candy, gumballs, Dum Dum pops)," mixed roughly equal amounts by weight of top, middle, and bottom tier candies, and threw them into the containers by the handful, in order to give the candy a natural spatial distribution.

Each container was filled to a capacity where it could be reasonably carried without spilling and then weighed on a hanging spring scale (adjusted to account for the weight of the container).

Their results"¦

A 10-quart bucket held a total of 9.5 lbs of candy, consisting of 375 pieces.
A standard white 5-gallon plastic bucket allowed for 20 lbs of candy in 675 pieces.
A double-bagged, regular brown paper grocery bag held 25 lbs of candy, consisting of 885 pieces. The researchers found that the bag's unreliable handles were problematic once the bag was full.
A standard size pillow case, allowing enough empty room at the top so that it may be grasped and picked up with two hands, held a whopping 47.75 lbs of candy in the form of 1690 pieces.

Next, they wanted to know if it would be possible to even collect that much candy in one night of trick-or-treating. How far would one need to walk and how many houses would they have to hit?

The researchers picked two different middle-class residential areas representative of suburban America at large to use in the experiment. Campbell, California, in Silicon Valley is an older area with dense housing, and St. Peters, Missouri, a suburb of St. Charles, is more rural and contains many newer developments. The researchers used data from to approximate the number of houses per square mile and constructed several different trick-or-treating scenarios, varying the values for the number of candies received at each house, and the percentage of houses distributing candy. In their worst case scenario, they figure a trick-or-treater would have a 50% success rate and receive an average of 2.5 pieces of candy per house, while a decent trick-or-treating run would see a 75% success rate and 3.5 pieces of candy per house.

They researchers then used Google maps to work out what sort of mileage a candy hunter would have to clock. Assuming the first scenario, a trick-or-treater would have to visit approximately 1352 houses and cover .42 square miles in Campbell, given the housing density, to fill their pillowcase. Under the more favorable conditions of the second scenario, it would take visits to 644 houses and .2 square miles to fill a pillowcase. Looking at the their map, the researchers estimated roughly 1 linear mile of street distance per every .036 square miles, meaning one would walk about 11 miles to fill their candy bag in the worst case scenario.

In the better scenario in St. Peters, the lower density of housing necessitates that someone cover .6 square miles to fill a pillowcase. That's more walking than in the worst case scenario in Campbell—and since the researchers' housing densities are based on statistical averages and don't account for undeveloped land, a trick-or-treater would likely need to cover a lot more ground. [Image courtesy of They've got some fabulous stuff on their site. Who among us hasn't wondered whether Viagra keeps flowers fresh?]


Begins and Ends: European Cities
Penn Vet Working Dog Center
Stones, Bones, and Wrecks
New Program Trains Dogs to Sniff Out Art Smugglers
Penn Vet Working Dog Center
Penn Vet Working Dog Center

Soon, the dogs you see sniffing out contraband at airports may not be searching for drugs or smuggled Spanish ham. They might be looking for stolen treasures.

K-9 Artifact Finders, a new collaboration between New Hampshire-based cultural heritage law firm Red Arch and the University of Pennsylvania, is training dogs to root out stolen antiquities looted from archaeological sites and museums. The dogs would be stopping them at borders before the items can be sold elsewhere on the black market.

The illegal antiquities trade nets more than $3 billion per year around the world, and trafficking hits countries dealing with ongoing conflict, like Syria and Iraq today, particularly hard. By one estimate, around half a million artifacts were stolen from museums and archaeological sites throughout Iraq between 2003 and 2005 alone. (Famously, the craft-supply chain Hobby Lobby was fined $3 million in 2017 for buying thousands of ancient artifacts looted from Iraq.) In Syria, the Islamic State has been known to loot and sell ancient artifacts including statues, jewelry, and art to fund its operations.

But the problem spans across the world. Between 2007 and 2016, U.S. Customs and Border Control discovered more than 7800 cultural artifacts in the U.S. looted from 30 different countries.

A yellow Lab sniffs a metal cage designed to train dogs on scent detection.
Penn Vet Working Dog Center

K-9 Artifact Finders is the brainchild of Rick St. Hilaire, the executive director of Red Arch. His non-profit firm researches cultural heritage property law and preservation policy, including studying archaeological site looting and antiquities trafficking. Back in 2015, St. Hilaire was reading an article about a working dog trained to sniff out electronics that was able to find USB drives, SD cards, and other data storage devices. He wondered, if dogs could be trained to identify the scents of inorganic materials that make up electronics, could they be trained to sniff out ancient pottery?

To find out, St. Hilaire tells Mental Floss, he contacted the Penn Vet Working Dog Center, a research and training center for detection dogs. In December 2017, Red Arch, the Working Dog Center, and the Penn Museum (which is providing the artifacts to train the dogs) launched K-9 Artifact Finders, and in late January 2018, the five dogs selected for the project began their training, starting with learning the distinct smell of ancient pottery.

“Our theory is, it is a porous material that’s going to have a lot more odor than, say, a metal,” says Cindy Otto, the executive director of the Penn Vet Working Dog Center and the project’s principal investigator.

As you might imagine, museum curators may not be keen on exposing fragile ancient materials to four Labrador retrievers and a German shepherd, and the Working Dog Center didn’t want to take any risks with the Penn Museum’s priceless artifacts. So instead of letting the dogs have free rein to sniff the materials themselves, the project is using cotton balls. The researchers seal the artifacts (broken shards of Syrian pottery) in airtight bags with a cotton ball for 72 hours, then ask the dogs to find the cotton balls in the lab. They’re being trained to disregard the smell of the cotton ball itself, the smell of the bag it was stored in, and ideally, the smell of modern-day pottery, eventually being able to zero in on the smell that distinguishes ancient pottery specifically.

A dog looks out over the metal "pinhweel" training mechanism.
Penn Vet Working Dog Center

“The dogs are responding well,” Otto tells Mental Floss, explaining that the training program is at the stage of "exposing them to the odor and having them recognize it.”

The dogs involved in the project were chosen for their calm-but-curious demeanors and sensitive noses (one also works as a drug-detection dog when she’s not training on pottery). They had to be motivated enough to want to hunt down the cotton balls, but not aggressive or easily distracted.

Right now, the dogs train three days a week, and will continue to work on their pottery-detection skills for the first stage of the project, which the researchers expect will last for the next nine months. Depending on how the first phase of the training goes, the researchers hope to be able to then take the dogs out into the field to see if they can find the odor of ancient pottery in real-life situations, like in suitcases, rather than in a laboratory setting. Eventually, they also hope to train the dogs on other types of objects, and perhaps even pinpoint the chemical signatures that make artifacts smell distinct.

Pottery-sniffing dogs won’t be showing up at airport customs or on shipping docks soon, but one day, they could be as common as drug-sniffing canines. If dogs can detect low blood sugar or find a tiny USB drive hidden in a house, surely they can figure out if you’re smuggling a sculpture made thousands of years ago in your suitcase.


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