After many hours of extremely acrobatic and exhausting sex with a blonde he had just picked up, a man goes into the kitchen for some food to replenish his just-spent energy.
He pours himself a glass of milk and right before drinking it, he realizes his manhood is still pretty hot, so he sticks it in the glass to cool it off.
Just then the blonde walks in and says, "Oh, I always wondered how you refilled those."