Mathigon
Mathigon: Infinity
Explore this course on infinity the including topics of the continuum hypothesis, Cantor's diagonal, and countability.
National Council of Teachers of Mathematics
The Math Forum: Ask Dr. Math: Rational Numbers
Math Forum response addressing how many elements are in the set of rational numbers. Responses investigate the countability of sets, and how sets with an infinite number of elements can be counted and compared to other "infinite sets."