Watch the video below of my year 7s performing an activity.

They are in a line numbered 1 to 25 and start by sitting down. When I call out a times table, everyone in that times table has to change what they are doing.  That is, if they are sitting they should stand and if they are standing they should sit.

If call out all the times tables from 1 to 25, who is left standing at the end? Why?

When we went back into the classroom we investigated why this had happened. I asked questions such as: 

How many times did (say) 24 stand up and sit down?

How many times did the square numbers stand up and sit down?

This was very fruitful and led to discussion about how factors come in pairs, and also that prime numbers only have two factors.

It was one of the best ways of showing students the structure of numbers and their factors I have ever used!