Can you solve the prisoner hat riddle? - Alex Gendler
TED-Ed
4 min, 35 sec
Ten captives must pass an alien test of logic to avoid being eaten by determining the colors of their hats.
Summary
- The alien overlords have challenged ten humans with a test to determine if they are logical and cooperative enough to avoid being eaten.
- The captives are lined up in size order and each given a black or white hat; they must guess the color of their own hat without any direct clues.
- The first person in the line communicates the parity of black hats he sees to help others deduce their hat colors.
- By using the parity information, each subsequent captive can deduce the color of their hat, with the group only allowed one incorrect guess.
Chapter 1

Captives are presented with a logic test by aliens who cannot eat highly logical beings.
- Ten individuals are captured by aliens who find humans tasty but cannot consume logical beings.
- The aliens are uncertain about humans' logical abilities and offer a test to prove their capacity for logic and cooperation.

Chapter 2

The aliens explain the rules of the test that will determine the captives' fate.
- The captives will be lined up in size order, unable to see behind them or leave the line.
- They will each have a randomly assigned black or white hat and must guess its color when prompted, without using signals or words other than 'black' or 'white'.
- If at least nine captives guess correctly, they will be spared; otherwise, they will be eaten.

Chapter 3

The captives must devise a strategy that uses limited communication to save themselves.
- The captives have five minutes to discuss a plan using the limited communication allowed by the aliens.
- The key to the strategy lies in the first person's ability to see all other hats and communicate coded information.

Chapter 4

The solution centers on using the parity of visible black hats to inform the others.
- The solution is to use the first person's statement to indicate the parity of the number of black hats he sees.
- By saying 'black' or 'white', the first person conveys whether the number of black hats ahead is odd or even, allowing others to deduce their hat color.

Chapter 5

The captives apply the agreed-upon strategy to determine their hat colors.
- The tallest captive starts by indicating the parity he observes, sacrificing his chance to guess correctly.
- Each captive uses the parity information and the previous guesses to deduce the color of their own hat.

Chapter 6

The strategy ensures the captives' survival, demonstrating their logical prowess.
- The strategy proves successful as it allows each captive, except the first, to guess their hat color with certainty.
- The parity rule ensures that the aliens cannot eat the captives due to their logical and cooperative behavior.

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