ABSTRACT
Philipp Frank was a good friend of Einstein's and visited him one time in the 1920s Einstein was somewhat miffed at some of the public criticism of his relativity theory in the press and at widespread assertions that relativity was difficult to understand. He told Frank that he had just written a short easy book on relativity that any layman could understand: in fact his twelve-year-old daughter had read the book and understood it. A little while later, when Einstein had left the room, Frank turned to Ilsa and asked her if she had really understood her father's book. 'Yes' she replied 'all except what he meant by inertial reference frames.'
