Sex change

A couple's taking a walk, and as they walk hand-in-hand, the guy starts to get aroused.
He's just about to get frisky when she says, "I hope you don't mind, but I have to take a pee."
He says, "Sure, go behind the hedge."
She disappears behind the hedge, and as he waits, he hears the sound of her jeans being rolled down her legs and imagines her exposed twat. He can't contain himself, so he reaches through a gap in the hedge and touches her leg. Then he moves his hand up her thigh until he finds himself gripping something long and thick between her legs.
He says, "My God, Mary, did you change your sex?"
She says, "No, I changed my mind. I'm taking a shit instead."