Here are two circles. One was drawn freehand with a pencil. The other was drawn with a compass.

Look closely: which one is a true circle, and what makes it perfect all the way round?

Watch three circles on the board, each labelled with its parts. Look for where the centre sits and how far the radius reaches. The circumference is the whole distance all the way around the edge. The diameter goes right across through the centre, and it is made of two radii laid end to end, so it is always exactly twice the radius.

Now a second circle, smaller than the first. The radius is shorter this time. Notice what has happened to the diameter now that the radius is shorter.

The last circle brings in two new names. One is a straight line joining two points on the edge, and the other is a piece of the edge itself. Watch where each one sits, and decide whether that straight line could be called a diameter.

Now you build circles to match a target on the screen. The first two targets give you the radius straight out, so you can set it and check. The last two give you the diameter instead, which means you have to halve it yourself before the compass goes anywhere near the right size.