Perfect Roundness

A circle is every point at exactly the same distance from a center. That distance is the radius. Double the radius and you get the diameter. This simple rule produces the most symmetrical shape possible — infinite lines of symmetry and infinite rotational symmetry.

A circle also packs the most area inside the shortest possible boundary. That is why bubbles pull themselves round and why nature reaches for circles and spheres whenever it needs to wrap the most space in the least edge.