
In response to not knowing the speed of sound as included in the Edison Test: New York Times (18 May 1921); Einstein: His Life and Times (1947) Philipp Frank, p. 185; Einstein, A Life (1996) by Denis Brian, p. 129; "Einstein Due Today" (February 2005) edited by József Illy, Manuscript 25-32 of the Einstein Paper Project; all previous sources as per Einstein His Life and Universe (2007) by Walter Isaacson, p. 299
Unsourced variants: "I never commit to memory anything that can easily be looked up in a book" and "Never memorize what you can look up in books." (The second version is found in "Recording the Experience" (10 June 2004) at The Library of Congress http://www.loc.gov/exhibits/world/world-record.html, but no citation to Einstein's writings is given).
1920s