Tom visits Dave who's laid up at home with a broken leg. Dave says "Me feet are freezing mate, can you nip upstairs and get my slippers?"
"No probs" says Tom.
Upstairs Dave's stunning 19 year old twin daughters are sitting on their bed. "Hello girls, your Dad sent me to shag you two."
"Fuck off you liar!" they said.
"I'll prove it." said Tom and he shouts downstairs "Both of them Dave?"
"Of course! What's the point of fucking one?!"