I thought this is a real cool problem… IT is all about logical thinking…

A teacher took out 8 stamps from a box – four of them white and four of them black. He got three of his brightest students – X, Y and Z and made them stand facing each other. He showed them the stamps, blindfolded each of them and then put two stamps on each of their foreheads. He put the final two stamps back in the box and closed the box. Then he opened the blindfolds. So, each of X, Y and Z could see the stamps on the forehead of the other two (but not their own – or the ones in the box). But they could obviously hear each other.

The teacher asked X first what were the color of the stamps (White White or Black Black or White Black) on his forehead. He said he did not know. The teacher asked then Y the same question. Y said he did not know either. Similarly the teacher asked Z and he too replied he did not know.

The teacher went back to X and asked if he now knew what he had. X again replied in the negative. The teacher asked Y next. And he did answer.

What was his answer? How did he deduce it?