‘Imagine having someone stay in your house for 40 days without smiling,’ my friend Mark said when his son was just a few weeks old. He spoke with the halting, far away speech of that guy in a horror movie who’s just seen his entire platoon eaten by giant centipedes. ‘Not one smile,’ he continued. ‘You’d think they were a psychopath.’

Only now do I realise how true this is. Of all the strange things about the small, soft man that has been sleeping in a box by my bed for the past six weeks, the weirdest is the fact he hasn’t smiled. It has, in fairness, been physically impossible for him to do so, as social smiling is a development that doesn’t kick in before now, but still – it’s hard not to take it personally.

If my son didn’t smile some of the time, I’d be OK with it. If he simply found his mother’s humour morbidly basic, for example, I would totally understand. I’ve never tried heroin, but I can only imagine it feels something like the rush my wife experiences when a bride falls through a wedding cake on You’ve Been Framed. On those rare occasions I leave her alone with my laptop, it takes weeks for YouTube to stop recommending me bloopers from The One Show, or rude words being spelled out on Countdown.

But my son doesn’t even laugh at me, and I’m a celebrated wag, famed for my charm and easy wit. Even my most delightful skits and routines garner the same reaction of bored alarm I get from friends when I explain the chronology of the Fast & Furious movies.

I know he’s a baby, but I don’t think a slight movement of the face is too much to ask, I’m not demanding he negotiate me cheaper car insurance, or write a revelatory new biography of William Pitt the Elder.

I’d just rather he didn’t endure the happiest moments of his life sporting the bobble-headed stare of a house party casualty on bad acid. Of course, his lucid moments are even more jarring, when his entire body jerks into motion after an hour spent gazing dejectedly at the ceiling. These flashes also come freighted with a sort of paranoid horror. Even as we comfort him, he glares at us with the dour, spiteful mien of an elderly dowager railing at the servant helping her into bed, convinced he’s planning to steal her many, and priceless, velvets.

But then it happened. Not with song, or play, or even a particularly satisfying 4am shite, but over a nonsensical chat one early morning. He smiled – actually grinned, from ear-to-ear – and a rush of pure ecstasy flew through us that was so strong, my wife considered it equivalent to 6,000 brides falling through the biggest wedding cake she’d ever seen.

I held him close and continued babbling. ‘The chronology of the Fast & Furious movies is actually straightforward,’ I whispered, eager to draw out the moment with my best material, ‘so let’s go back to the third film in the series, Tokyo Drift, where things do get slightly complex.’

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