There are 3 students lined up one behind the other, facing the front of the room. The teacher has 3 black hats and 2 white hats. The teacher puts 1 hat on each student randomly, and tells them whoever can say for sure what their hat color is gets an A for the semester.
The students can see the hats in front of them, but not their own of course. And they do not know what the remaining 2 hats are.
After some time, the front most student (ie. the one who can’t see any of the others heads), declares, “I have a black hat.” And he was completely right. How did he know?