Naturally, being itself a set, the set of all sets would also have to contain itself as an element. Russell's Paradox ...

In this lesson, students learn to find the fraction of a set. In this lesson, Ms. Brewer will teach 3rd grade math students to find the fraction of a set. / Carrie Brewer, East Saint Louis School ...

Called ZFC, or Zermelo-Fraenkel set theory (C stands for “axiom of choice”), the system is a collection of basic assumptions sufficient to carry out almost all of mathematics.