"MY baby's going to be baby Jesus," announced Jonah just before the New Year. These days, he speaks with an air of great authority, no matter what he's saying.

He'd learned all about Baby Jesus this Christmas, from the moment he was chosen to play Joseph in the church nativity play.

Not that he was the obvious candidate. 'Mary' was six years old, tall and serious. Jonah's just three, and half her size. But there was a severe shortage of boys to choose from, so he got the part.

He was thrilled to bits. When I said to him on the phone: "I hear you're going to be Joseph in a play," he corrected me: "I am Joseph."

I wasn't able to be there, but I've a feeling the actual performance on the Sunday before Christmas wasn't as much fun for Jonah as thinking and talking about it. There was a lot of waiting around beforehand, and he didn't like wearing the usual tea towel-style headdress. But it went well enough, with all the usual opportunities for parental laughter and tears, and there were some good photos for us to enjoy.

But that wasn't the end of it as far as Jonah was concerned. He now knows what Christmas is all about. He could recognise all the figures in the Christmas cribs he saw, in our house, in Fenwick's window. And he liked to play at being Joseph, acting out the story over and over again. At bathtime, he got his mum to play Mary and carefully wrapped the plastic toy penguin in 'swaddling clothes' (a facecloth).

So maybe it's not surprising he believes 'his' baby is going to be the baby Jesus, though quite what he means by that I'm not sure. Maybe it's just that he thinks this baby will be around to play the infant in next year's nativity play, though I doubt if he can really think that far ahead. In any case, I'm not sure how real it all is to him.

He must have noticed that his mum was getting a bit fat, but he'd always thought you got a fat tummy by eating too much. Then at last she told him he was going to have a baby brother and it was in her tummy.

We'd all wondered how he'd react when he was told. With excitement? With wonder? With jealousy? His mum even worried in case he asked difficult questions about how it got there. He did none of those things. He laughed. He thought the idea of a baby being inside his mummy's tummy was the funniest thing he'd heard in a long time.

He's got used to it now. He accepts it as fact. He does know a bit about brothers and sisters, because some of his friends at nursery have them. But I doubt all the same that he really understands what all this is going to mean.

Ever since we first knew about the baby we've all been doing our best to make Jonah think positively about babies, long before he himself was told.

There was a tiny baby cradled in its mother's arms at the family party we went to in September. It seemed like a good opportunity to get Jonah interested. "Look at the little baby," I said to him, and was pleased to see how attentively he gazed at it.

It was a very young baby, with tiny dimpled hands and no more than a faint fuzz of hair. After considering it for quite a long time, Jonah said: "The baby's been to the hairdressers."

So he knows what babies look like. He knows Christmas is about baby Jesus. He knows there's a baby in mummy's tummy. He talks a lot about "my baby" as if it's important to him, a sort of special present just for him.

But I guess it'll still be something of a surprise, even a shock, when in a few weeks time, his little brother arrives in the world.

On the other hand, he knows that in the Christmas story Joseph had to help look after baby Jesus. Perhaps that will help him understand that his little brother will need caring for too. It could be a very good place to start.