Actually, gerry, there's sometimes another explanation. If before putting on your shoes you walk around in your socks, sometimes a little stone or grit on the floor sticks to the sole of your sock. If it's in the middle of your foot, where it curves upward, you don't notice it when you put on your shoes; but you do if it then works its way to your heel or to the ball of your foot as you walk along. So possibly the answer is not always as obvious as robber 1 thinks, though quite often it is.