With a little bit of delay, another classic puzzle today.
Four prisoners are lined up in a row. A wall separates the last prisoner from the first three, so that the fourth prisoner cannot see the others, and the first three prisoners can only see forward. The prisoners know that each of them is wearing either a black or white hat, and that there are two hats of each color.
The prisoners can only make one guess about the color of their own hats. If they guess correctly, they will all go free. If anyone guesses wrong, they will all remain in prison.
Starting from the back, each prisoner in turn (from fourth to first) is asked if they know the color of their hat. Only one of them says, "Yes." Which prisoner speaks, and what color is their hat?
Previous iteration: #756004 (several correct answers in the thread)