Cellular automata are grids of cells which can be different colours. The colours update each turn based on how the grid is currently coloured and some set of rules. They are normally generated by computer, but originally they were done with pencil and paper.
The most famous cellular automaton is probably Conway's Game of Life. This uses a 2D gird of cells which can be black or white (alive or dead).
I thought it might be interesting to try to make one with my five year because they involve following simple rules and can generate interesting patterns. I decided to use a triangular grid because the colouring rule then only uses two cells.
How it works
First you need a triangular grid, which you can printed from here.
Then colour the top square. We used green because it was approaching Christmas and it looked a bit like a Christmas tree.
Then, go to the row below (with two squares) and colour any square that has exactly one coloured square above (which will be both of them in this case).
Keep working your way down, row by row, following this rule until you have done all the rows.
Where is the maths
The first thing I like about this activity is that we are using the plus operator on colours. It shows how mathematics is about define some some simple rules and seeing what happens.
We could replace the empty square with 0 and the green square with 1 to get binary arithmetic. Or we can start with 1 and then use normal arithmetic. This generates Pascal's triangle, which is full of interesting properties. If you colour the odd numbers, then you will generate the same Sierpiński triangle.
My five year old quite enjoyed this project, but kept checking with me that he was doing it right. I probably should have just let him do what he thought was best and see what happened, but when I tried that, he just missed out regions. He also got a bit bored by the end, and didn't finish it. But, since it was it was nearly Christmas, he did decorate it.
Before I explained what we were going to do, my son found the page with two grids I'd printed out and started colouring in cells. He made it so the grids where reflections on one another, because he'd been doing symmetry at school. It looks like there's a worthwhile activity in there somewhere.