Imagine you are putting a line of fairy lights right around the edge of the class notice board, or a fence all the way around a garden. How would you work out how much you need to buy?

The distance all the way around the edge has a name in maths. Today we find it for lots of different shapes.

Look at these three shapes on the board — each one has its side lengths marked. Let's walk around each edge and add as we go to find the perimeter, counting every side once.

The next one is a square. Look at its perimeter before we reveal it — is it more or less than the rectangle?

The last one bends around a corner. A shape made of simpler shapes joined together — like an L — is called a composite shape. Its perimeter is still just all its outer sides added up, so here you have to add all six sides, not just the outside two.

One of you builds each shape on the board while the rest of the class adds the sides aloud. We start with a 5 by 3 rectangle, then stretch it into a 7 by 2 one. Before anyone reads the answer off the screen, say the sum together — one side at a time, right around the shape.

Now measure for real. Find the perimeter of a desk top and your maths book with a metre stick or ruler. Then take the printed sheet your teacher gives you and measure the three shapes on it: a triangle, a five-sided shape and an L-shape. For each one, measure every side, add them up, and record the perimeter in your copy.