An (infinite) set C is said to be countable if there exists a bijection between C and the set N of natural numbers. This means that the elements of C can be listed as c1, c2, c3, ···so that every element of C will appear, sooner or later, in the list. For example, Z is countable, because you can list all the integers in the order 0, 1,-1, 2,-2, 3,-3, ···. Is the set Q of all rational numbers countable? How about R and C?