Question: Suppose you comb your hair with a part on the left side of your head. When you look in a mirror you will observe that the part is now on the right side of the head looking back at you. You will also observe that your head is still at the top and your feet are still at the bottom. If you lie down horizontally on your side and look into a mirror you will observe that the mirror image of your hair parting is still switched left to right. Why does a mirror reverse left and right but not up and down?
From: Ace the Programming Interview [Guiness]