70 year old man goes into a brothel. He picks out a young pretty woman, Long they go up to her room, strip down and climb into bed. The old man performs like a teenager, the prostitute is amazed at how energetic and agile he is, she tells him if he can do it like that again, she'll give him one for free. He says "Yeah, I can, but I need to take a 20 minute nap, and while I'm asleep, I need you to hold my old pecker." She agrees, he wakes up 20 minutes later and goes at it again, just as vigorously as before. The girl is amazed at the old man's stamina, and repeats her freebie offer, the old man tells her that once again, he'll need a 20 minute nap and she'll have to hold his dick while he's asleep. She does as he asks, he wakes up 20 minutes later and he goes at it again, with even more enthusiasm than previously.
The hooker catches her breath, and needing to satisfy her curiosity, asks the old man "I can understand why you need the nap, but why do you need me to hold your dick while you're sleeping?" The old man replies "Oh, that's just so you don't steal my wallet.