My greatest lesson isn't mine; it belongs to my old teacher, David Meek, who taught English at Boclair Academy in Glasgow.
He had three memorable features: a beard (then unusual; oh, how I hope he lived to see the renaissance of chin merkins), an evangelical faith and an inability to tolerate sloppy thinking. He was one of the first teachers I ever had who didn't let us swots wallow in our fortresses of smug but made us work for gold stars. He challenged us to think and write, to debate and argue. You rarely saw his like.
And he taught this lesson. I can't even remember how it fitted into the syllabus, but this was 1987, when presumably you could teach a class on Mork amp; Mindy if the Muse seized you. The lesson was on probability. And I never forgot it.
He started at the board. Counting up the pupils, he wrote "30".
"That's how many of you are in the room," he said. We looked around and some of us counted, too.
"By the age of 40," he continued, "25 of you will be married." We looked around, wondering if we'd all marry each other. "Statistically speaking, it's probable that you will be. And statistically speaking we can say quite a lot about your probable futures.
"By the same age, 12 of you will be divorced." This was more serious. While not rare, divorce was still noteworthy in 1987. These were the last few years of shame. The News of the World could still run scandalous stories about gays and affairs and saucy vicars having gay affairs. Tradition still dominated conformity like Godzilla in the Chewits advert.
We started to enjoy the game. "Seven of you will have left the country," he went on. "Six of you will be unemployed. One half of one of you will be a millionaire."
And on and on. Like a magician turning three cherries in a tin into a banana, he turned the classroom into the whole world - and the future.
Then the trick changed. "Three of you will have cancer. Six of you will have less than 20 years to live. Five of you will have had a serious accident. And one of you will be dead." Everyone in the room froze.
"I'm not telling you that to make you sad," Mr Meek said. "I'm telling you that to make you live. I want you to appreciate that, young as you are, youth will run out and life will happen whatever you do. So do as much as you can, while you can."
This was two years before Dead Poets Society, in which Robin Williams' character John Keating whispered carpe diem to his privileged New England cohort. But the film resonated because teachers like Mr Meek had been saying the same things for decades, centuries even: gather ye rosebuds while ye may. Make hay while the sun shines, for time is fleeting. Over the years, we married, divorced, and fell in and out of work. When one of us died a few years later, we remembered that lesson.
I certainly never forgot. I use the same activity from time to time with my classes. The statistics change, as do the topics, but I still see Mr Meek, reflected in the children's eyes. And what he said still haunts me.
Tom Bennett teaches at the Jo Richardson Community School in Essex and is director of the ResearchED conference