Killer Puzzle

For the last homework of the year, I gave my students a killer puzzle—literally. Puzzle. A mysterious man kidnaps people, takes them to his cabin, and offers each victim two identical-looking pills. He claims that one pill is poisonous and the other is harmless. The victim chooses one pill, swallows it with water, and dies, while the killer consumes the other pill and survives. How does the killer manage to get the safe pill every time? The official answer was that neither pill was poisoned. The poison was in the water. Some students suggested that the poison becomes active only when mixed with water. I did not give full credit for this solution: saliva contains water, so the proposed chemistry is off. Naturally, my students suggested other solutions. Here are two good ones, which are similar to each other: Both pills are poisonous, but the killer has taken an antidote. Both pills are poisonous, but the killer has somehow built up a resistance to the poison. And here is an ingenious…