18 in 20 minutes!

A cop was patrolling at night in a well-known spot for "parking." He sees a couple in a car, with the interior light on. He gets closer to the car and sees a young man behind the wheel, reading a computer magazine and a young woman on the rear seat, knitting. Puzzled by this surprising situation, the cop walks to the car and knocks at the window. The young man lowers his window...
Yes, officer?

What are you doing?

Well, isn't it obvious? I'm reading a magazine...

Pointing towards the young woman, the cop says: And her, what is she doing?

The young man shrugs: I believe she's knitting a pullover...

The cop is totally confused. A young couple alone in a car at night... and nothing obscene is happening! What's your age, young man?

I'm 25, sir...

And her, what's her age?

The young man looks at his watch and says: "She'll be 18 in 20 minutes..."