Three tourists climbed up the tower with London's Big Ben and decided to throw their watches off the top, run down the stairs and try to catch them before they hit the ground. The first tourist threw his watch but heard it crash before the had taken three steps. the second threw his watch and made only two steps before hearing his watch shatter. The third tourist threw his watch off the tower, went down the stairs, bought a snack at a shop up the street and walked slowly back to Big Ben in time to catch the watch." How did you do that?" asked one of his friends.

"My watch is 30 minutes slow."