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9 of History’s Best Riddles

1. A Hobbit Head Scratcher

Anyone who’s gotten lost in Middle Earth knows that J.R.R. Tolkien loved a logic puzzle. The riddle competition between Bilbo Baggins and Gollum in The Hobbit serves up several mind-bending morsels, the trickiest of which might be: 

Voiceless it cries,
Wingless flutters,
Toothless bites,
Mouthless mutters. 

Answer: The wind 

2. The Mad Hatter’s Dirty Trick 

One of the most famous literary riddles in literature is also the most frustrating ... because it came without an answer! In Alice's Adventures in Wonderland, the Mad Hatter poses this puzzle to Alice: 

“Why is a raven like a writing desk?” 

Answer: The Hatter doesn’t have the answer, and as it turns out, Carroll didn’t, either. But readers’ desire for closure was so intense that the author was forced to dream up an answer that later appeared in a preface: 

Enquiries have been so often addressed to me, as to whether any answer to the Hatter's Riddle can be imagined, that I may as well put on record here what seems to me to be a fairly appropriate answer, viz: 'Because it can produce a few notes, tho they are very flat; and it is never put with the wrong end in front!' This, however, is merely an afterthought; the Riddle, as originally invented, had no answer at all.”

3. Oedipus’s Complex Problem 

In Sophocles’ Oedipus Rex, the title character must answer to the Sphinx to save his own life and continue his journey to Thebes. Spoiler: he nails it. The monster asks: 

What walks on four feet in the morning, two in the afternoon and three at night? 

Answer: “Man: as an infant, he crawls on all fours; as an adult, he walks on two legs and; in old age, he uses a 'walking' stick.” 

4. A Harry Potter Puzzler 

The Harry Potter series is teeming with playful language and cleverness, so it’s only right that a juicy riddle made its way into the series. In The Goblet of Fire, J.K. Rowling gives a nod to the Sphinx by putting one in the maze during the Triwizard Tournament. Harry is tasked with cracking this puzzle:

First think of the person who lives in disguise,
Who deals in secrets and tells naught but lies.
Next, tell me what’s always the last thing to mend,
The middle of middle and end of the end?
And finally give me the sound often heard
During the search for a hard-to-find word.
Now string them together, and answer me this,
Which creature would you be unwilling to kiss?

Answer: A spider.

5. Guarded Truths 

The riddle was coined by mathematician Raymond Smullyan and goes by many names—“A Fork in the Road,” “Heaven and Hell,” and “The Two Doors,” among them. It is probably most well known for having a role in the 1986 movie Labyrinth. Here’s the basic idea: You’re met with a choice between two identical doors with an identical guard at each. One door leads to heaven and one door leads to hell. You can ask one guard one question and then make your choice on which door to pass through. One of the guards always tells the truth and one of them always lies. So, what question do you ask? 

Answer: In Labyrinth, the protagonist (Sarah, played by Jennifer Connelly) gets it right. She asks the one on the left, “Would he [referencing the guard on the right] tell me that this door leads to the castle?” Leftie tells Sarah yes, and from there, she is able to conclude that he is the one guarding the door to “certain death.” This can get tricky to work through, but luckily the Internet has an unending supply of resources if you want a deep dive into the puzzle’s logic.

6. A Bully Riddle 

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This riddle was rumored to be Teddy Roosevelt’s favorite: 

I talk, but I do not speak my mind
I hear words, but I do not listen to thoughts
When I wake, all see me
When I sleep, all hear me
Many heads are on my shoulders
Many hands are at my feet
The strongest steel cannot break my visage
But the softest whisper can destroy me
The quietest whimper can be heard.

Answer: An actor 

7. James Joyce Goes Deep 

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In Ulysses, Stephen Dedalus poses a riddle to his pupils. A word to the wise: don’t spend too much time trying to work this one out. 

The cock crew,
The sky was blue:
The bells in heaven
Were striking eleven.
‘Tis time for this poor soul
To go to heaven.”

Answer: “The fox burying his grandmother under a hollybush.” 

Get it? Dedalus’s students don’t, and many scholars believe that’s sort of the point. The exaggerated difficulty is meant to be a kind of riddle about riddles. 

However, not all of Joyce’s riddles in Ulysses are impossible. Protagonist Leopold Bloom jokes, "Good puzzle would be cross Dublin without passing a pub.” This equally baffling head scratcher was solved by a software developer in 2011. The programmer managed to map all of Dublin’s pubs and used an algorithm to chart a course that never comes within 115 feet of one. 

8. The One That Started It All 

There is debate over who wrote the first riddle, but the ancient civilization of Sumer is certainly responsible for one of them. Sumerians’ contribution to the legacy of logic problems: 

There is a house. One enters it blind and comes out seeing. What is it?

Answer: A school 

9. Think Hard 

Another oldie-but-goodie originated in 18th century England, though you might know it from Die Hard with a Vengeance

As I was going to St. Ives,
I met a man with seven wives,
Each wife had seven sacks,
Each sack had seven cats,
Each cat had seven kits:
Kits, cats, sacks, and wives,
How many were there going to St. Ives? 

Answer: One. As John McClane learns, this is a classic trick question. If the narrator meets the group on the way to St. Ives, then they must be going in the opposite direction and the math calculations are simply a bit of trickery meant to misdirect

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iStock // Ekaterina Minaeva
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Man Buys Two Metric Tons of LEGO Bricks; Sorts Them Via Machine Learning
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iStock // Ekaterina Minaeva

Jacques Mattheij made a small, but awesome, mistake. He went on eBay one evening and bid on a bunch of bulk LEGO brick auctions, then went to sleep. Upon waking, he discovered that he was the high bidder on many, and was now the proud owner of two tons of LEGO bricks. (This is about 4400 pounds.) He wrote, "[L]esson 1: if you win almost all bids you are bidding too high."

Mattheij had noticed that bulk, unsorted bricks sell for something like €10/kilogram, whereas sets are roughly €40/kg and rare parts go for up to €100/kg. Much of the value of the bricks is in their sorting. If he could reduce the entropy of these bins of unsorted bricks, he could make a tidy profit. While many people do this work by hand, the problem is enormous—just the kind of challenge for a computer. Mattheij writes:

There are 38000+ shapes and there are 100+ possible shades of color (you can roughly tell how old someone is by asking them what lego colors they remember from their youth).

In the following months, Mattheij built a proof-of-concept sorting system using, of course, LEGO. He broke the problem down into a series of sub-problems (including "feeding LEGO reliably from a hopper is surprisingly hard," one of those facts of nature that will stymie even the best system design). After tinkering with the prototype at length, he expanded the system to a surprisingly complex system of conveyer belts (powered by a home treadmill), various pieces of cabinetry, and "copious quantities of crazy glue."

Here's a video showing the current system running at low speed:

The key part of the system was running the bricks past a camera paired with a computer running a neural net-based image classifier. That allows the computer (when sufficiently trained on brick images) to recognize bricks and thus categorize them by color, shape, or other parameters. Remember that as bricks pass by, they can be in any orientation, can be dirty, can even be stuck to other pieces. So having a flexible software system is key to recognizing—in a fraction of a second—what a given brick is, in order to sort it out. When a match is found, a jet of compressed air pops the piece off the conveyer belt and into a waiting bin.

After much experimentation, Mattheij rewrote the software (several times in fact) to accomplish a variety of basic tasks. At its core, the system takes images from a webcam and feeds them to a neural network to do the classification. Of course, the neural net needs to be "trained" by showing it lots of images, and telling it what those images represent. Mattheij's breakthrough was allowing the machine to effectively train itself, with guidance: Running pieces through allows the system to take its own photos, make a guess, and build on that guess. As long as Mattheij corrects the incorrect guesses, he ends up with a decent (and self-reinforcing) corpus of training data. As the machine continues running, it can rack up more training, allowing it to recognize a broad variety of pieces on the fly.

Here's another video, focusing on how the pieces move on conveyer belts (running at slow speed so puny humans can follow). You can also see the air jets in action:

In an email interview, Mattheij told Mental Floss that the system currently sorts LEGO bricks into more than 50 categories. It can also be run in a color-sorting mode to bin the parts across 12 color groups. (Thus at present you'd likely do a two-pass sort on the bricks: once for shape, then a separate pass for color.) He continues to refine the system, with a focus on making its recognition abilities faster. At some point down the line, he plans to make the software portion open source. You're on your own as far as building conveyer belts, bins, and so forth.

Check out Mattheij's writeup in two parts for more information. It starts with an overview of the story, followed up with a deep dive on the software. He's also tweeting about the project (among other things). And if you look around a bit, you'll find bulk LEGO brick auctions online—it's definitely a thing!

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Why Your iPhone Doesn't Always Show You the 'Decline Call' Button
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When you get an incoming call to your iPhone, the options that light up your screen aren't always the same. Sometimes you have the option to decline a call, and sometimes you only see a slider that allows you to answer, without an option to send the caller straight to voicemail. Why the difference?

A while back, Business Insider tracked down the answer to this conundrum of modern communication, and the answer turns out to be fairly simple.

If you get a call while your phone is locked, you’ll see the "slide to answer" button. In order to decline the call, you have to double-tap the power button on the top of the phone.

If your phone is unlocked, however, the screen that appears during an incoming call is different. You’ll see the two buttons, "accept" or "decline."

Either way, you get the options to set a reminder to call that person back or to immediately send them a text message. ("Dad, stop calling me at work, it’s 9 a.m.!")

[h/t Business Insider]

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