Shown in the image below are 4 men buried up to their necks in the ground. They cannot move so can only look forward. Between A and B is a brick wall which can not be seen through. They know that between them are 4 hats, 2 are black and 2 are white, but they do not know which colour they are wearing.
In order to avoid being shot one of them must call out to the executioner the colour of their hat. If they get it wrong, everyone will be shot. They are not allowed to talk to each other and have 10 minutes to fathom it out.
Man 'D' can see both the hats in front of him, btw.
Which man calls out, and what does he say?
(don't post the answer if you know it please.)