We present to the attention of the young, and not so young, reader a book series composed of the once widely known works of the talented domestic popularizer of science Yakov Isidorovich Perelman. The book that begins the series, which you now hold in your hands, was written by the author in the 20s of the last century. Immediately becoming extremely popular, it has been virtually out of print since then and is now very rare. The book is devoted to questions of mathematics. It contains a variety of mathematical puzzles, many of which are presented in the form of short stories. This book, as Ya. I. Perelman said, “is intended not for those who know everything that is generally known, but for those to whom it has yet to become known.” All books in the series are written in the form of a casual conversation, including original calculations, successful comparisons to stimulate scientific creativity, illustrated by a motley series of puzzles, intricate questions, entertaining stories, amusing problems, paradoxes and unexpected parallels. The author's writing style has been preserved unchanged; The statistical data presented in the book correspond to the 20s of the twentieth century.