You probably have some idea of what *infinity* means. Something that is infinite goes on forever. But it might surprise you to know that there are different kinds of infinity, and some are bigger than others!

As an example, a circle has an infinite number of points. (Yet the circumference of the circle has finite length.) Compare that to a straight line with *infinite* length. Both have infinitely many points but does the finite length circle have fewer? [The answer is *no*.]

To understand all this, we have to first talk a bit about numbers.