On one side of the wall you have a guy with a black hat. He can't see over the wall.
On the other side of the wall, you have three guys in a line with hats. The one closest to the wall has a white hat. The middle one has a black hat. The last one has a white hat. They can't turn around. The 4 also can't say anything unless they know with 100% certainty what color hat they have on. There can only be 2 black hats and 2 white hats. The last one in line can see the two people ahead of him, so he knows which color hats they have on.
Which one knows with absolute certainty which color hat he has on?
(No cheating, internet savvy forum members!)
*Also, share some brain busters of your own!