Watch the compass go all the way round in one smooth turn and draw a perfect circle. What did the compass keep the same the whole way round, and what changed?

Here are three finished circles drawn with a compass. Each was made by setting the gap to the radius, planting the sharp point, then rotating the pencil-arm without lifting it.

Now a bigger circle. Notice the gap between the point and the pencil is the radius every time.

And a small circle, where the two legs are almost touching. Look carefully at how the curve joins back up.

Today we draw a circle to each radius we call out: first 3 cm, then 5 cm, then 2 cm. The screen lets you set the radius with the slider and draw; you set your own compass gap against your own ruler, plant the point, and rotate without lifting. One of you works the screen at the front while everyone draws the same circle on paper.

Today we draw a circle to each target radius: 2.5 cm, then 3 cm, then 4.5 cm, then 5 cm. Measure each gap against your ruler before you plant the point, then check your circle against the target on screen.