Three men, Alan, Brian and Colin are blindfolded and are told that either a red hat or a green hat will be placed on each of their heads. After this is done, the blindfolds are removed and the men are asked to do the following:
· To raise a hand if they see a red hat.
· To leave the room as soon as they are sure of the colour of their own hat.
All the hats happen to be red, so all the men raise a hand. Several minutes elapse until Colin, who is more astute than the others, leaves the room. How did he deduce the colour of his hat?
C asks himself, “Can my hat be green?” If so then A will know immediately that he has a red hat, for only a red hat on his head would cause B to lift his hand. A would have therefore left the room. B would reason in the same way and also leave. Since neither has left the room, C deduces that his own hat must be red.