This is to a class of eight and nine-year-olds and I want the lesson to do one thing - involve them in focused discussion so I can find out what they know and build on it.
A lot of hands shoot skywards, and the answers I get are revealing about what the children think.
"Because of tens and units?"
"There are 10 millimetres in a centimetre."
"It's an easy number."
So far, all the answers have confused cause with effect, giving examples of outcomes of a base-ten counting system, not reasons for having one.
But eventually, and after some very florid and exaggerated hand-gesturing as I move around the room, someone suggests it could be because we have 10 fingers.
Quite quickly, everyone agrees, and now the fun can start. Together we go back to a time when spoken language is just beginning and writing is just a dream in the future.
We're scouting for mammoths. We haven't eaten for days. Suddenly, out of a snow flurry, there they are - six of them. We run back to the cave, but how can we tell our news to the others waiting there?
Now, there's a dramatic interlude. Small groups of children work together,deciding how best to relay the message, and the answer is amusing, but effective.
We mime running back to the cave, picking up weapons - there's a lot of digging and stabbing. Elephant impersonations abound, and bunches of six fingers stab at the air.
We talk some more. Yes, it's quite easy to use fingers to pass on numbers greater than 10, but the bigger the number, the more confusing it gets.
"You could use your toes as well."
It takes us a while to negotiate this one, but the consensus is that, what with its being an Ice Age, it's risky to take off our fur boots.
Now I can lead up to the idea of choosing an object to represent 10 fingers - a pebble perhaps?
The children get the idea quickly, and there's no holding us back now. Three pebbles and four fingers - we'll never be hungry again.
What happens when the elk migration passes this way in the spring? There are so many of them. Lots of pebbles - pouches full.
There follows some more discussion, focused on how we can avoid carrying all these extra pebbles. "You could find some twigs."
"Well, every twig could be 10 pebbles, like every pebble is 10 fingers."
We're on a roll.
By the end of the lesson, we are experimenting with ways of writing down our messages, and some children are complaining that it takes a long time to draw all these pebbles and twigs. "Isn't there a shorter way?"
But that's another lesson.
Mel Williams is headteacher of St John's primary school, Wallingford, Oxfordshire