What tool do you use to draw a perfect circle?

The instrument used to construct (or draw) circles is called a pair of compasses, although it is often known either as compasses or simply as a compass.

A pair of compasses usually consists of a joined pair of "legs", one of which has a sharp point, and the other of which holds a pencil. The sharp point is pushed lightly into the paper and acts as a pivot, while the other leg can be swung around it to draw an arc, or a complete circle.