The best thing about mathematical puzzles is the moment when the light bulb goes on - when the right idea strikes and leads you to the solution without much calculation. This book offers over 200 opportunities for this rewarding experience. For these brain teasers, often just common sense is enough; sometimes, basic knowledge of algebra, geometry, or number theory is required. And then there are those problems that are unsolvable - which you only realize once the proof of unsolvability is found! But that is, after all, a solution too!.