Sometimes teenagers are very bad-tempered. If it's coming up to a mealtime, there's no mystery. They're like lions in the zoo pacing around waiting to be thrown half a deer or a bucketful of cow.
But at other times, you haven't got a clue what's going on. You walk on eggshells.
You might be able to guess why your teenager is wandering about with a face like thunder. Perhaps pressure is being piled on at school because of exams. Or a friend is behaving badly. Or they haven't got any money (and neither have you, so it's all completely hopeless).
But sometimes you really don't know what the problem is.
"Everything I say, he bites my head off," said a friend recently. "And if I ask him what's wrong he stares at me as if I'm stupid." She was looking completely miserable.
You have to be careful, as a parent. You have to watch like a hawk for signs of serious difficulty like depression. I read an article about young men and suicide which made me want to wrap my sons in cotton wool and never let them out of the house.
But most of the time your maternal anxiety doesn't unearth any deep underlying problems. Your teenager is just in a bad mood. You have to wait until the storm passes.
But there is a silver lining. I was leaving my friend's house recently when her son, the source of all her despair, came in from a night out. I stood there, slightly nervous. After all, I'd heard a lot about shouting matches and slamming doors.
He looked up. He gave me a brilliant smile. "Hello," he said. "How are you?"
Your teenager may be horrible at home. But to the outside world, he's completely and utterly lovely.