Old (VERY old) joke,
A cop is walking around lovers lane and comes up to a parked car. He taps on the window and the boy rolls it down. The cop asks, "What are you two doing?", "Well she's knitting and I'm reading the paper.", "And how old is your girlfriend?" asks the cop. "17 Sir." The cop then asks "And when's her birthday?" The boy looks at his watch and says "In about 10 minutes."