We bring to your attention the next, fourth, book by the popular Russian scientist and teacher Ya. I. Perelman. It is composed of two now little-known works of the 20s of the last century: “Tricks and Entertainment” and “Box of Riddles and Tricks.” The author appears before us in an unusual capacity - a magician and sorcerer. He gives his reader the opportunity to see amazing tricks, then revealing their mathematical secrets. The amazed reader sees extraordinary and “wonderful” things, which, as it later turns out, are based on simple arithmetic calculations.Ya. I. Perelman collected interesting experiments and tricks that amazed those around him, to perform which you will need the most ordinary objects that are always at hand. All this will certainly arouse your and your child’s interest in the exact sciences and brighten up your leisure time. These tricks are “honest and conscientious”, and, showing intelligence and the ability to reason, everyone can do them. You will learn something that others don't even know about. And by showing them to your friends and acquaintances, you can work wonders like a professional magician. You will amaze the imagination of your viewers, turning into a mathematical genius before their eyes. The author's writing style has been preserved unchanged; The statistical data presented in the book correspond to the first half of the 20th century.