This one's for age 13 up - to help with equations. It's a cute trick I dreamed up today, because when I'm doing maths, I like to make up stories and I love to draw.
This story is about a magic mirror. It's for when you're working with 'balancing' equations, where [something] = [something] and you need to shift chunks from one side onto the other. When I explained it to my friend's daughter - who's about to take her GCSEs - she started out freaked out and ended up laughing.
I've drawn a dotted line through the centre of the equations to show you how this magic disappearing trick is done.
Every time you push something over to the other side, the sign (+ or -) reverses - just like when things are reflected in a mirror.
When all the bits and bobs have vanished, you're left with zero, which is perfect for when you need to start juggling with the bits - factorising, simplifying, and what have you.