The earliest evidence of this saying located was printed in a newspaper article on December 12, 1930 by David P. Sentner who was a correspondent for the International News Service (INS). Albert Einstein was a passenger aboard the Red Star Liner Belgenland which docked in New York, and Sentner reported on the questions that were addressed to Einstein by a group of reporters. One query concerned his remarkable theory of relativity:

“Can you explain your theory simply for the masses of America?” he was asked.
“No,” he answered running his fingers through his shock of graying hair, “it would take me three days to do it.”