A small child stops at a puddle on the pavement; she has somewhere to be — school, perhaps, or lunch, or the firm trajectory of a parent's hand — and yet she stops, because the puddle is reflecting the sky, and the sky is doing something extraordinary at her feet, and she does not, at the age of four, yet know any of the things one is supposed to know about light and water and surface tension. She knows only that something marvellous is happening down there, and that, for the moment, she has all the time in the world for it.

I want to ask the question that hides inside that small scene, because I think it is more searching than it sounds at first hearing — when, exactly, did you stop having time for puddles? It is a strange enquiry, but worth following, because somewhere along the long road from there to here, most of us decided, almost without noticing, that the puddle could wait, or was beneath our notice, or that stopping for it would make us late, or look foolish, or seem the sort of person who has not yet quite grown up; and the cost of those decisions, accumulated quietly over decades, may turn out to have been larger than we have ever paused to calculate.

What does growing up cost us, when it is done a little too thoroughly? The answer, for so many of the people I have known into their fourth and fifth decades, is wonder — the capacity to be surprised, the willingness to be the only person on the high street looking at the sky, the unselfconscious pleasure of doing a thing for no reason whatsoever other than that the thing is pleasurable. Is it possible that one has, somewhere along the way, mistaken seriousness for substance, and dignity for depth, and the careful maintenance of an image for actual living?

There is a peculiar dignity-disease that settles, almost imperceptibly, over many of us in our thirties and forties — we become custodians of an impression rather than inhabitants of a life; we grow careful, we notice ourselves being watched, we stop dancing at weddings because no one else is dancing yet, we stop singing in the kitchen on the off chance someone might hear, we stop laughing in public at things which are, in fact, genuinely funny, and we begin instead to perform a small, contained, polite smile — which is the laughter equivalent of clearing one's throat. And one is forced to wonder, with a certain rueful tenderness, when one's laughter became something closer to a polite cough; and what, exactly, one was protecting oneself from when one made that subtle and almost imperceptible exchange.

I do not, I should say, want to suggest that maturity is the enemy. We need the adult, who pays the bills and sets the boundaries and calms the room when something has gone wrong; the adult is necessary, and not to be done away with. But the adult is not the whole of the self, and the adult on her own is rather dry, and the adult on her own is, on the long view, exhausting to live with — particularly when the person you happen to live with is yourself.

What, then, is play, exactly, in the technical sense? It is, I think, the doing of a thing without a purpose; that is rather its definition; and as soon as a thing acquires a purpose — once it is being tracked on an app, optimised, justified, productivised — it has quietly ceased to be play and has become work in lighter clothing. This is why so many of our supposed leisure activities turn out, on inspection, not to be leisure at all: the hobby that must be improved at, the yoga practice with a streak counter, the reading-list that must be completed, the holiday with its meticulous agenda. Real play, by contrast, is permitted to be useless — and when, exactly, was the last time you allowed yourself to do something genuinely, unapologetically useless, with no improvement plan and no anticipated outcome? When was the last time you painted something badly at the kitchen table, knowing perfectly well you would never show it to anyone? When did you last build something out of pillows, or invent a ridiculous voice for a cat who is plainly not amused?

The shame around playfulness in adults is, I have come to suspect, the shame of someone who has been told too many times to be sensible — it is the inherited shame of pretending we are not, beneath the suits and the spreadsheets, deeply silly creatures who require silliness in approximately the same way we require sleep. We are not, despite our most strenuous efforts, computational beings; we are not adults made entirely of work; we are mammals with mammal needs, and one of those needs is the rough delighted play we last allowed ourselves around the age of nine, and have been quietly missing ever since.

What does playfulness restore, when one lets it back in? It restores, I think, the part of you that is permitted to be wrong — the part that does not have to be competent, the part that can attempt the thing and miss it and try again, because there is no audience and the stakes are nothing. Most of our adult anxiety, when one looks closely, is the anxiety of performance — the performance of intelligence, of competence, of having it all together — and play is the one place where being bad at something is genuinely all right, because the badness was never the point in the first place. Could you, then, be cheerfully bad at something this week, on purpose, and let yourself enjoy the badness? Could you sing in the car at the volume the song actually requires, even if your voice cracks rather embarrassingly on the high note? Could you spend half an hour at the kitchen table with a small child — a niece, a granddaughter, a neighbour's daughter — and let her be the boss of the game, and play her game by her rules, and notice something loosen in your chest that you had not realised, until just that moment, had been so tightly bound?

There is, I think, a particular joy in being silly with someone who can keep up — and good friendships and good partnerships, on the long arc, are made of this far more than they are made of sober conversation; they are made of the unguarded licence to be daft together, of the private nickname that would embarrass you in public, of the same joke repeated for the seventeenth time and still landing, of the knowledge that this person has, on multiple occasions, seen you absolutely absurd, and likes you better for it. Who, in your life, has earned the right to see you absurd? And if the answer is "nobody", perhaps that is the first thing worth attending to.

The art of being playful is not, you see, about pretending we are not adults; it is about refusing the false choice between adulthood and aliveness, about insisting that the grown-up get to keep, in some small protected corner, the child she once was — the one who stopped at the puddle, the one who sang in the bath, the one who laughed until she could not breathe. She is still in there, somewhere, and she is, I rather suspect, bored, and she is waiting. Would you let her out today, even just for ten minutes? Would you, on the way home, stop at the puddle? The sky is reflecting in it, only for a moment; the errands will wait; and the grown-up world will not, on this one occasion, collapse if you spend a minute looking at the sky in the wrong place.