Lights will go on, illuminating the whole house, and then off again. You will hear footsteps thumping up the stairs. Irritated at being disturbed, you sigh nonetheless. All is well. You go to sleep.
The next morning, querulous, you say, 'Can't you come back quietly? Do you have to wake the whole household?'
But just when you think you understand how the system works, you realise you know nothing at all.
On Saturday night, my daughter went out to see the fireworks in our local park. At home, because we're old and boring, we stayed in and watched a DVD. Then we went to bed.
At 2am, I woke in a panic. Where was she? Terrified, I blundered into her room. There she was, in bed, fast asleep.
The next morning I said, 'What time did you get back last night?'
My daughter said, yawning, 'About midnight.'
I wanted to say, but I didn't hear you come in! I didn't know you were safe! Anything could have happened! I could have been lying there, cuddled up in the duvet, lost in happy dreams, while you were out in the cold and dark battling with terrible danger!
I said, 'I didn't hear the front door.'
She smiled, sleepily. 'I made sure I was quiet,' she said.
So there you have it. You teach your teenagers to behave with thoughtfulness and consideration. And when they do, you realise it's not what you wanted at all.