Articles about puzzles

One Hundred Prisoners and a Light Bulb — van Ditmarsch & Kooi

You and two other logicians (Alice and Bob) are in a room. A controller comes in and paints a spot onto each of your foreheads. You can each see the others’ spots (Alice and Bob both have black spots) but not your own. The controller tells you all that all the spots are black or white, and at least one of you has a black spot. Then the controller asks if anyone knows the colour of their spot. Everyone says no. The controller asks the same question a second time: again, everyone says no. The controller then asks the same question a third time. What do you say now?

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