Four men are standing in a line as shown above and they cannot turn their heads. There is a brick wall between A and B that they cannot see through. They know that they each have one hat. Two of the four hats are black and the rest are white. They cannot see the color of the hat that they are wearing. In order to be released from3 the line, they must call out the color of their hat. If they are wrong, no one can leave. They are not allowed to talk to each other, and they have been given ten minutes to figure out their hat color. After 1 minute:
·One of them calls out—which one called out?
·How can he be 100% certain of the color of his hat?
This is not a trick question4 and there are no outside influences nor any other ways of communicating. They cannot move or see behind them, nor can they see through the wall.

Answer:  C would call out, because he would know that if his hat were the same color as B’s hat, then D would have known that his own hat was the opposite color of B and C’s hat and would have called out. Since D did not speak then C knows that his hat is the opposite color of B’s.

1. rottweiler   n. 德国洛威纳犬，非常凶猛
2. sure enough 果真；的确
3. be released from 从…解脱出来
4. trick question 题面上带有陷阱的问题，如脑筋急转弯之类。