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Can You Solve the Three Gods Riddle?

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In this TED-Ed riddle, we explore a classic logic puzzle invented by logician Raymond Smullyan. As the video tells us, it has been called the hardest logic puzzle ever. They're not kidding.

So here's the situation. You have crash-landed on a mysterious planet. The only way to escape is to appease three alien overlords (they were "gods" in the original telling, hence its name).

You know that the three aliens are named Tee, Eff, and Arr. There are also three artifacts on the planet, each of which matches a single alien (for the sake of simplicity, let's assume they are labeled "Tee," "Eff," and "Arr"). To appease the aliens, you need to match up the artifacts with their aliens—but you don't know which of the aliens is which!

You are allowed to ask three yes-or-no questions, each addressed to any one alien. (You can address multiple questions to the same alien, but you don't have to.)

To further complicate things, each alien has a specific behavior with regard to telling the truth. Tee's answers are always true. Eff's answers are always false. Arr's answers are random.

Yet another problem is that while you know the alien words "ozo" and "ulu" somehow correspond to "yes" and "no," but you don't know which is which. (I know, this situation just keeps getting worse!) So while you can communicate enough of the alien language to ask questions, they will respond only with "ozo" or "ulu." You may ask the questions one at a time, building on each response if you wish—meaning you have time to think about the next question based on what you have learned.

So your task is to ask three yes-or-no questions, while not knowing which answering words correspond to "yes" and "no," of three aliens whose identity is unknown, but whose behavior is predictable...if you knew their identity. How can you figure out which alien is which, so you can hand the right objects to the aliens? Put your thinking caps on, then roll the video explanation (which includes the answer, after a suitable pause):

For more on this puzzle, check out this TED-Ed page. For some flavor on logician Raymond Smullyan, check out his interview with Johnny Carson.

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Watch How Computers Perform Optical Character Recognition
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Optical Character Recognition (OCR) is the key technology in scanning books, signs, and all other real-world texts into digital form. OCR is all about identifying a picture of written language (or set of letters, numbers, glyphs, you name it) and sorting out what specific characters are in there.

OCR is a hard computer science problem, though you wouldn't know it from its current pervasive presence in consumer software. Today, you can point a smartphone at a document, or a sign in a national park, and instantly get a pretty accurate OCR read-out...and even a translation. It has taken decades of research to reach this point.

Beyond the obvious problems—telling a lowercase "L" apart from the number "1," for instance—there are deep problems associated with OCR. For one thing, the system needs to figure out what font is in use. For another, it needs to sort out what language the writing is in, as that will radically affect the set of characters it can expect to see together. This gets especially weird when a single photo contains multiple fonts and languages. Fortunately, computer scientists are awesome.

In this Computerphile video, Professor Steve Simske (University of Nottingham) walks us through some of the key computer science challenges involved with OCR, showing common solutions by drawing them out on paper. Tune in and learn how this impressive technology really works:

A somewhat related challenge, also featuring Simske, is "security printing" and "crazy text." Check out this Computerphile video examining those computer science problems, for another peek into how computers see (and generate) text and imagery.

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Can You Solve This Fish Riddle?
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Transporting cargo by boat doesn’t usually require solving tricky brainteasers. That’s not the case with this fishy riddle from TED-Ed.

For this scenario, imagine you're a cargo boat director who’s charged with shipping several tanks of rare fish to an aquarium. The tanks are tossed overboard during a rough storm and it’s your job to retrieve them. There’s a mini-sub onboard that might be of assistance, but there’s a problem: You only have enough fuel for it to make one quick trip. Before launching your rescue mission, you need to figure out exactly how many tanks fell into the water and where they landed.

After referring to sonar data, thermal imaging, and your shipping notes, you come up with this list of information to help narrow down your search.

1. There are three sectors where the cargo landed.

2. There are 50 animals in the area, including the lost fish and deadly sharks.

3. Each sector contains between one and seven sharks and no two sectors have the same amount of sharks.

4. The tanks each have the same amount of fish.

5. There are 13 tanks at most.

6. The first sector has two sharks and four tanks in it.

7. The second sector has four sharks and two tanks.

So how many fish are there? If you came up with 39, you’re right. There can only be 39 fish spread out across 13 tanks, which means there are three fish in each.

In case you’re feeling more confused now than you were before, you can refer to TED-Ed’s full explanation in the video below.


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