Maybe a quite easy question. Why is s1 s 1 the unit circle and s2 s 2 is the unit sphere? Also why is s1 ร—s1 s 1 ร— s 1 a torus?

In mathematics, the unit circle is a circle with a radius of one. Frequently, especially in trigonometry and geometry, the unit circle is the circle of radius one centered at the origin (0,0) in the cartesian รขโ‚ฌยฆ