"Last Marshmellow" joke

A man is urinating one day when the end of his penis drops off. He thinks, "This is probably not a good thing," so he picks up the knobby end and sticks it in his pocket, then races off to the doctor. He waits in the surgery for a bit, then he's called in.

The doctor greets him and asks, "What's the problem?"

"Well, doctor, I was urinating and my knob fell off. Here it is." And he reaches into his pocket and hands the piece to the doctor.

The doctor looks, frowns, then replies, "What are you talking about? This is a marshmellow!"

"Well, that can't be right! I ate my last marshmellow on the way in here!"