A piece of cake. Say that you and a mathematician want to split a single piece of cake. You each want a fair share, so you agree that the mathematician will slice the cake and you will choose the half you want. The mathematician cuts it down the middle and you take the piece that you think is slightly larger. Now the mathematician feels like he has gotten an equal portion -- he cut it in half, remember -- and, because you were given a choice, you believe that you have gotten slightly more than he got. Miraculously, the two halves will seem to add up to more than a whole