Toes
A couple were indulged in sexual intercourse and the man noticed that with each movement of his pelvis, his partner's toes would rise.
Later that night, while going at it pretty hot and heavy in the shower, her toes remained still. Confused, he asked, "Why is it that when we do it in bed, your toes go up, but when we do it in the shower, they don't?"
"Silly," she replied, "I take my pantyhose off in the shower!"