Remember, when someone annoys you, it takes 42 muscles in your face to frown, but it only takes four muscles to extend your arm and smack the asshole in the head.
A woman enrolled in nursing school is attending an anatomy class. The subject of the day is involuntary muscles. The instructor, hoping to perk up the students a bit, asks the woman if she knows what her asshole does when she has an orgasm.
"Sure!" she says, "He's at home taking care of the kids ..."