The teacher drew a line from the center to the edge of her wobbly circle and wrote 'r' over it. "The distance from the center of a circle to part its outside is called the radius. When we do calculations with the radius, we let 'r' stand in for it because writing 'radius' every time would get annoying. There's a radius for every circle, so in this case, let's say the circle has a radius of four." 