For reasons which may never be known, four men wearing two black and two white caps line up on either side of a wall as shown below, (the colour of the cap on each man's head is for illustration purposes only). Each man knows that there are two black caps and two white caps but he can see only the colour(s) of the cap(s) on the  man or men in front of him. Nobody can see the colour of his own cap. The men can only look forwards and can not see through the wall. They can not talk to each other either but they may speak to their foreman. The foreman asks them to shout out the colour of their cap but if the answer is wrong that person will be put on heavy duties. If the answer is correct that person will be allowed a ten minute break. None of the four men want to do heavy duties but, after a short while, one of the men correctly gives the answer. Which man answered  and how did he know the colour of his cap?

Give in? Want to check your answer? Follow the link to the solution.