Once upon a time, there was a sperm named George who lived inside a famous movie actor.
George was a very healthy sperm. He'd do push ups and somersaults and limber himself up all the time, while the other sperm just lazed around doing nothing. One day, one of the sperm questioned George and asked why he exercised all day. George explained, "Look pal, only one sperm gets a woman pregnant, and when the right time comes, I'm gonna be that one sperm!"
A few days later, all the sperm could feel themselves getting hotter and hotter. They knew the big swim was imminent.
Moments later, they were released abruptly and, sure enough, George was swimming far ahead of all the others. Suddenly, George stopped in his tracks, turned around, and began to swim back with all his might. "Go back! Go back!" he hollered, "It's a blow job!"