Pretty much every high school student can quote the formula for the area of a circle: it's A = pi r^2, where r is the radius. Most of them can do the same thing for the volume of a sphere: V = 4/3 pi r^3.
You could, if you wanted, generalize the concept and call circles "2-spheres", because they're the set of all points a given distance from the center in a 2d space. A sphere would be a "3-…