Bob was walking home one day when he passed a man jumping up and down on a manhole cover and saying "Sixteen...sixteen...sixteen..." with each jump. Bob was curious, so he asked the man what he was doing.
"Tell you when," the man said. "You go up that hill over there, and run down here straight at me. If you can tackle me off this manhole, I'll tell you everything."
Bob had been a footballer in his youth and took the bet. He raced down the hill at the skinny man and, just before he plowed the skinny guy off the manhole, he jumped back and opened the manhole cover. Bob fell, screaming, into the darkness.
The skinny man replaced the manhole cover, stepped back onto it and began jumping again.