Three visitors to London climb up the tower that houses Big Ben and decide to have a contest. They're going to throw their watches off the top, run down the stairs and try to catch the watches before they hit the ground.

The first tourist throws his watch, takes three steps and hears his watch crash. The second throws his watch and takes only two steps when he hears his watch shatter.

The third tosses his watch off the tower, jogs down the stairs, goes to a candy store, buys a snack, walks back to Big Ben and catches his watch. "How did you do that?" asks one of his friends.

"My watch is 30 minutes slow."