Look at these three round things: a clock face, a bicycle wheel and a coin. Where is the very middle of each circle? And if you wanted to measure straight across the widest part, where would you put your ruler?

Watch the circle on the board. First we see the centre — the exact middle. Then a straight line from the centre out to the edge: that line is the radius. When a second radius points the opposite way, the two together make one straight line right across the circle through the middle: that is the diameter. When we have more than one radius, we call them radii: so one radius, two radii. Notice how far the diameter reaches compared to a single radius.

Today we fold a paper circle to find its parts. Each of you has a circle. Follow these three steps:

1. Fold your circle in half so the two edges land exactly on top of each other. Open it: that crease runs right across through the middle, so it is a diameter.
2. Fold again a different way and open it. Where the two creases cross is the centre.
3. Point to a radius: from that crossing point straight out to the edge.

We take turns at the board to make these circles. Before each one, the whole class says the missing measurement out loud. The board tool is set by its radius, so for a diameter target we first halve it to find the radius to set. Say the missing measurement for each:

• radius 4 cm
• diameter 10 cm — what radius?
• radius 3 cm
• diameter 12 cm — what radius?