There's one great pleasure in being a grandparent that doesn't get much of a mention.

Most of the joys are obvious enough. Someone once said that having grandchildren is like falling in love. There's a lot of truth in that - except that falling in love tends to be a phase that either develops into something deeper and more enduring, or fizzles out in boredom or disagreement.

Loving grandchildren is for life. And (though I'm pretty new to it) so is the total obsessiveness. Everyone knows how grandparents always seem to have a pile of photographs stashed away in a handy pocket to bring out with tedious predictability on every possible occasion.

Then there's the longing to be with the grandchildren as much as is humanly possible. Even though we live hundreds of miles away from them, we still seize every chance to go and see them. Our hearts leap at the sight of them. We love giving them treats - small 'un-birthday' gifts (who needs an excuse?), trips to the zoo. We push their buggies with pride through the streets and delight in the feel of the little hand clutching our own.

We even enjoy the tougher bits of the job. Because one of the best things about being grandparents is being able to give something very special to their parents, your own children - the gift of your time. We love being able to take them off their parents' hands, whether for a couple of hours, or even, occasionally, for a night or a whole weekend. Sometimes just taking charge of the baby monitor for a night or two is the best gift you can offer exhausted parents.

And at the end of it? Well, tired but content, you hand them back to their parents and go home, looking forward to when you can see them again.

And that's when the rarely mentioned pleasure of grandparenthood comes into the picture.

If you've cared for them for 24 hours or so, you'll be exhausted. You've had lots of fun, but you've also had nothing but infant conversation for all that time. There have been no breaks; you've thought of nothing but the little ones. It's been like going back to your own days as new parents, though you had a lot more energy then, if rather less patience.

But it's soon over and suddenly you're just the two of you again, adults doing adult things. The little ones are safe and happy, and you are free. Free to sleep without interruption, free to talk when you want about what you want, free to enjoy adult meals in civilised comfort.

That's the point when, after visiting the grandchildren in London, we often take ourselves off to a restaurant. Once, scruffy and exhausted, we treated ourselves to lunch in Selfridges, in a swanky, expensive bistro place. It was full of elegant people chatting politely over their food. We were waited on by courteous waitresses. The food tasted wonderful. There was not a child in sight. And it took us right back to those times when, as young parents, we'd leave our own children with my mother for an evening or a weekend and enjoy the snatched pleasure of a break from them. It felt naughty but delightful.

Now, that's what I call a pleasure - when you can relive the past and savour its joys, without having to do more than touch on its pains and difficulties. Hooray for grandparenthood.