There's a rule in London where you don't make eye contact.
If you see someone on the tube, or in the street, you studiously survey your shoes, or the nearest lamppost - anything - rather than lock eyes with the stranger opposite.
It's actually a sign of respect: population density is so high that your personal space is continually invaded. Everyone is so uncomfortably close at all times that the very existence of others feels aggressive. They're all up in your face. So the only way to tolerate it is to pretend that they - and you - don't exist.
But at that moment when it was over twenty degrees outside and easily forty inside, the rules changed.
It was a little after six on the train from London Bridge, and it had been a hard day. I was rushing around, over-caffeinated and under-fed. The carriage was dank and oppressive, filled with sweating bodies, each a picture of misery. I crammed myself in the best I could, dodging elbows and knees (and, mostly, armpits).
I clutched onto something - anything - as the train moved on.
The temperature rose further - past oven-hot into something else.
I don't know what happened next, but everything went dark.
As my legs gave way, I felt something else: hands behind me. Firm, but gentle, insistent, guiding me into place. A chair. Their chair.
A stranger had given up their seat on a crowded commuter seat at rush hour to make sure that I could sit down. A sudden wet-cold in my hand, and a bottle had appeared: another stranger had handed me a bottle of water.
Nodding mutely in appreciation, I began to drink.
My energy returned and, muttering profuse thanks, I left the train.
The journey took seven minutes, but the lesson lasted a lifetime:
Our neighbours aren't just the people next door, but the harried strangers we pass without a second glance.
Each act of kindness needn't be huge, but we each have it in our power to make the world a better place and to look out for each other.
My gratitude to those strangers, in that instance, has lasted over a decade. As the temperatures soar today, we can all be open to opportunities to really make a difference in someone's life.