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Imagine a matchstick.  
How many more are needed to make a square? How many more need adding to make yet another square along side it? Carry on adding more squares ... 

How many matches have you added? How many matches are there when you have made 10 squares in the row? 20 squares? 50 squares? 
We can arrange dots in a similar way to the 5 on a dice and they usually sit quite well into a rectangular shape. How many altogether in this 3 by 5? What happens for other sizes?
Compare the numbers of particular tiles in one or all of these three designs, inspired by the floor tiles of a church in Cambridge.
These alphabet bricks are painted in a special way. A is on one brick, B on two bricks, and so on. How many bricks will be painted by the time they have got to other letters of the alphabet?