My son started secondary school in September, which brought with it the challenge of taking the public bus across a chunk of north London.One afternoon in his first week, he didn’t turn up home — and he’d left his phone behind so I couldn’t contact him. All I could do was sit and wait.

So I sat, and I waited…

Eventually, I got a call from an unfamiliar number. It was him. “I’m at a dog rescue charity shop in Muswell Hill,” he said. “It’s a long story,” he added.

When I arrived at the shop in question, he was sitting there sipping juice and chatting with the staff. It turned out that he’d become confused about the bus route, got off in Muswell Hill and gone into the shop to ask for help.

“I love dogs,” he called over his shoulder as we left — which was a complete lie, but a well-intentioned one.

It turned out, when I asked for more details, that this was not the first dog rescue charity shop he’d visited that day. He’d also been to one on a different high street, from where he’d try to phone me, but I hadn’t answered.

“Why did you keep choosing dog rescue charity shops,” I asked.

“I thought it would be better for the story,” he replied — meaning it would make a funny anecdote to tell his friends about the next day. Truly, he is his mother’s son.

The point of writing about this at the end of the first week of Donald Trump’s presidency, is that my child was helped out by the kindness of strangers. Being kind to strangers (on a global level) is the absolute opposite of what Trump believes in, and it therefore seems particularly important to aim to be so in however small a way, whenever the occasion arises.

When the same child was a baby, I strapped him in the car ready to go shopping. Slamming the door shut, I realised, with horror, that I’d locked the keys in the car with him.

The spare key was in my in-laws’ house (we shared the car), and they were on holiday. I couldn’t go round and get it myself, because that would mean leaving the baby alone.

So I started to knock on the doors of neighbours I knew, to ask for help. But no one was in. No one was in! I clearly remember the feeling of rising desperation as I ran from house to house.

Eventually, I went to the house of a neighbour I didn’t know at all. A middle-aged lady answered the door.

“Do you have a car?” I demanded.

“Um… yes,” she replied, cautiously.

I explained the situation to her and said that I needed her to get in it, drive to my in-laws’ house, let herself in (I gave her their house keys), fetch our spare car key and bring it back to me.

“Right,” she said, and off she went. No fuss, no hesitation. Within 15 minutes she was back, and I had the car open. The baby didn’t even notice anything was amiss.

I will never forget it.

(It goes without saying that giving complete strangers the keys to your husband’s parents’ house is not on the official, how-to-be-a- good-daughter-in-law list. It’s also true, however, that the likelihood of middle-aged ladies who live in semi-detached houses in Finchley turning out to be dastardly criminals is pretty small.)

This is the grandest thing a stranger has ever done for me, but very often it’s the small acts of kindness that make a big difference to one’s day.

I was walking through the local park with my four-year-old last summer. He was being whingey and I was feeling harassed. An elderly lady came up to us.

“Do you know how to kick a pebble,” she said to him, and she picked one up and kicked it up the path. “You try,” she said.

He ran up and gave it a kick, beaming with delight, his mood totally transformed.

A bit later on, we bumped into her again and my son ran up to show her a new pebble he’d found — even better for kicking.

“Can I have first kick,” she asked him, but he quickly kicked it before she had the chance to, and they both laughed.

Alexander McCall Smith refers more than once in his books to the idea of “moral proximity”. In Friends, Lovers, Chocolate he writes: “We cannot choose the situations in which we become involved in this life; we are caught up in them whether we like it or not.

“If one encounters the need of another, because of who one happens to be, or where one happens to find oneself, and one is in a position to help, then one should do so. It is as simple as that.”

@susanreuben