Written for high school and college mathematics teachers and their students, as well as a general audience interested in mathematical ideas but lacking a formal background in the subject, this book begins by asking and answering the question "What is a number?". It discusses such concepts as positive integers, rational numbers, real numbers, complex numbers, polynomials, and the algebra on a dial. The book offers an introduction to algebra, including a discussion on groups and their actions, and linear algebra. Finally, it elucidates some basic properties of two- and three-dimensional geometry and topology. Thus, all the main branches of mathematics are addressed.