When they are born, that is. It is one of those basic little facts we always ask of a little bundle of joy.
"Boy or girl? Have you got a name yet? What about their weight?" we'll ask grinning away at a newborn.
And then something happens. We stop asking. Even parents stop paying attention to where their weans are on the scales.
Yet how much our young weigh is just as important as when they start talking or crawling or walking - or reading and writing.
It is a fundamental and important measure of well-being. Why? Because fat kills, sometimes quickly, sometimes slowly. And because it reinforces good habits of healthy eating and exercise for life.
Recently health staff have started weighing kids more often.
The result of this - and general improvements in diet and exercise - is slimmer children.
Today we reveal fewer are needing acute care for obesity. Good news, of course.
But it's too soon to celebrate.
And we all know the grim truth of fizzy drinks and snacking for kids stuck in front of the TV or video game console.
So let's all do our kids a favour. Let's keep asking their weight long after they are out of nappies.