When my oldest son Isaac was five, the big boy who lived in the house behind ours climbed over on to the roof of our shed and jumped down into the garden.

When I say “big boy”, he was all of nine years-old — and his visit had been planned. He and Isaac had arranged it by calling to each other over the fence.

His unorthodox method of arrival made a deep impression on our small son. Every now and then over the subsequent years, he has said: “Do you remember the time that boy who lives at the back climbed over on to our shed?”

He repeated it the other day, and his sister Emily (who would have been two at the time) said, “Yes! I do!”

“No you don’t,” said Isaac.

“I do,” insisted Emily.

This exchange was repeated over and over, following the rather dreary pattern common to so many siblings’ conversations, with both parties becoming ever more entrenched in their original position.

I was struck by a thought as I listened. Emily is utterly convinced that she remembers the incident — and it’s entirely possible that she does. But it’s also possible Isaac has told the story so many times, that Emily’s memory is of it being told, not of the event itself. There is no way of knowing — so it’s a good job that it really doesn’t matter.

One of my heroes, Douglas Adams, used to explain that he thought of the idea for The Hitchhiker’s Guide to the Galaxy while lying drunk in a field near Innsbruck, holding a copy of The Hitchhiker’s Guide to Europe. He claimed, though, that he had repeated this anecdote so often he no longer remembered the actual occurrence — only the telling of it.

It’s impossible to preserve a memory in its pure form. As you grow older, you see it through the eyes of the person you have become, and then the next time it rises to the surface, it has been subtly altered by the previous occasion. Layer upon layer of experience is added, making the original event ever harder to access.

Now and again, the memory of a long-ago moment will come unexpectedly to my mind — something that, as far as it’s possible to determine, I’ve never once thought about since it happened.

It’s usually something mundane, its very ordinariness meaning that it had stayed buried until a chance association brought it to light.

The memory arises fresh and undistorted. For perhaps a second, I am transported, as though I’ve truly become once again the person I was back then.

Then almost immediately the feeling is lost, as a thousand other thoughts crowd into my mind — among them reflections on the phenomenon I’ve just experienced.

For that one second, though, it feels as though I have travelled in time.

Another challenge with trying to achieve an authentic recall of the past is that memories tend to become fixed and rigid. Every time we think about an event in our lives, the same aspects of it come to mind.

Photo and film, supposedly an aid to memory, are often culprits in this regard. When I think of my wedding, I find it harder to bring back my experience of the day itself than I do to see it as a series of scenes from the official video. And I never film my kids’ performances at school plays and concerts, preferring to keep an imperfect record in my mind than to dull the experience by having half an eye on how I’m positioning my camera.

More poignantly, the memories of our loved ones who have died become fixed, too. I have recently connected, through social media, with the educator Doreen Samuels — an old family friend who knew my grandpa, my father’s father. He was a flamboyant character, raised in Dublin, who spent his career as one of the many Jewish doctors practising in Sunderland. Doreen told me a few funny stories about him that we had never heard — tales filled with whisky drinking, jokes and astute one-liners. “I hope these light up your memories,” she said. And indeed they did.

When we think about someone we’ve lost, we remember the same anecdotes, the same incidents and conversations. Even if the pool of memory is large, it rarely changes. So when a person comes along and tells us something new, it’s like a gift.

I realise that I’ve now left myself with a problem. I always show my kids in advance what I’ve written about them, and they’re allowed to veto it if they wish. If you’re reading this, you can assume that they gave the shed anecdote the thumbs up — but it will reignite the argument between the two of them about whether Emily recalls the boy climbing over.

“You don’t remember it!”

“Yes I do!”

“You don’t!”

I’m not sure I can bear it.

@susanreuben