The present edition of "Entertaining Physics" can be considered a kind of anniversary: the book appears for the tenth time and at the same time in the twentieth year of its existence. "Entertaining Physics" was born 20 years ago and was the firstborn in the large book family of its author, now numbering about forty members. The finished manuscript was handed over to the set in the spring of 1911, but lay motionless in the printing house for two years, since neither the publisher nor the author attached serious importance to it. Not without surprise, the author received friendly praise from reviewers when, in the spring of 1913, the book was finally published; especially encouraging was the benevolent assessment of such an expert as prof. O. D. Hvolson...