"Patience" joke

A cop was patrolling at night in a well-known spot for "parking." He saw a couple in a car with the interior light on. He got closer to the car and saw a young man behind the wheel, reading a computer magazine, and a young woman on the rear seat, knitting.

Puzzled by this situation, the cop walked to the car and knocked on the window.

The young man lowered his window... "Yes, officer?"

"What are you doing?"

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

Pointing toward the young woman, the cop asked, "And her, what is she doing?"

The young man shrugged. "I believe she's knitting a pullover."

The cop was totally confused. A young couple alone in a car at night... and nothing obscene is happening!

"What's your age, young man?"

"I'm 22, sir."

"And her, what's her age?"

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