A man and a woman meet in an elevator. "Where are you heading today?" the man asks.

"I'm going down to give blood."

"How much do you get paid for giving blood?"

"About $20."

"Wow," says the man, "I'm going up to donate sperm, and the sperm bank pays $100." The woman angrily gets off the elevator.

The next day, the man and woman meet in the elevator again.

"Fancy meeting you again. Where you off to today?"

"Sperm bank," she says with her mouth full.