A kid is in his bedroom, near ecstasy as he masturbates ever more vigorously. In walks his father.
"You'd better stop that, son," says the dad. "Otherwise, you're liable to go blind."
The boy dutifully affirms that he will stop immediately.
A week later, the dad once again walks in on the boy as he is masturbating.
"I thought we had an agreement," dad sputters angrily.
"Well," says the kid, "I figured I'd just quit when I need glasses."