You see, when a blind beetle crawls over the surface of a globe he doesn't notice that the track he has covered is curved. I was lucky enough to have spotted it.
Source, in full
Attributed to Einstein in Carl Seelig's Albert Einstein: A Documentary Biography (1956), p. 80. Said to have been a comment he made to his son Eduard when Eduard asked him, at age 9, "Why are you actually so famous, papa?"