From the preface of Aha! Solutions:
Every mathematician ( beginner, amateur, and professional alike ) thrills to find simple, elegant solutions to seemingly difficult problems. Such happy resolutions are called "aha! solutions," a phrase popularized by mathematics and science writer Martin Gardner in his books Aha! Gotcha and Aha! Insight. Aha! solutions are surprising, stunning, and scintillating: they reveal the beauty of mathematics. This book is a collection of problems whose aha! solutions I enjoy and hope you will enjoy too. The problems are at the level of the college mathematics student, but there should be something of interest for the high school student, the teacher of mathematics, the "math fan," and anyone else who loves mathematical challenges.