When your children are small, you sit with them and watch Disney classics like Bambi and 101 Dalmations.
When they're a bit older, you sit with them and watch family classics like Harry Potter and ET.
When your children become teenagers, you enter the nightmare world of totally inappropriate viewing.
To begin with, no 13-year-old is like any other 13-year-old. Some can only cope with PGs. Others are fine with 15s.
It's just as hard trying to find family viewing on TV. In theory, no material unsuitable for children should be shown before 9pm.
But straight after the watershed, there's suddenly a bare bottom moving up and down like a piston engine.
"Do you want a cup of tea?" says my son hastily.
"No, I'll make it," I say.
We have a brief fight in the doorway and collide in the kitchen, breathing heavily at a narrow escape.
TV crime dramas always have that moment when the tired detective has to track down an informant in a sleazy strip club. Suddenly there's pole-dancing and bare breasts.
Oh, the agony. I'm torn between burying my head in the cushions, hot with embarrassment, or shouting out protests about totally unnecessary shots of naked women.
If it's embarrassing for parents, it's excruciating for teenagers. The other day, The Inbetweeners (certificate 15) came on TV.
Within seconds, knowing that we were about to see a teenage boy in diving goggles watching porn, my daughter slid out of the room.
"Where are you going?" I say.
"I just remembered something," she said.
Probably that it's never a good idea to watch TV with your mum and dad.