“Cuando un hombre se sienta con una chica bonita durante una hora, parece que fuese un minuto. Pero déjalo que se siente en una estufa caliente durante un minuto y le parecerá más de una hora. Eso es relatividad.”

Original

When a man sits with a pretty girl for an hour, it seems like a minute. But let him sit on a hot stove for a minute and it's longer than any hour. That's relativity.

An explanation of relativity which he gave to his secretary Helen Dukas to convey to non-scientists and reporters, as quoted in Best Quotes of '54, '55, l56 (1957) by James B. Simpson; also in Expandable Quotable Einstein (2005) edited by Alice Calaprice

William Hermanns recorded a series of four conversations he had with Einstein and published them in his book Einstein and the Poet (1983), quoting Einstein saying this variant in a 1948 conversation: "To simplify the concept of relativity, I always use the following example: if you sit with a girl on a garden bench and the moon is shining, then for you the hour will be a minute. However, if you sit on a hot stove, the minute will be an hour." ( p. 87 http://books.google.com/books?id=QXCyjj6T5ZUC&lpg=PP1&pg=PA87#v=onepage&q&f=false)

In the 1985 book Einstein in America, Jamie Sayen wrote "Einstein devised the following explanation for her [Helen Dukas] to give when asked to explain relativity: An hour sitting with a pretty girl on a park bench passes like a minute, but a minute sitting on a hot stove seems like an hour." ( p. 130 http://books.google.com/books?ei=yma3TsDWK8WciQL63smAAQ&ct=book-thumbnail&id=vs3aAAAAMAAJ&dq=sayen+%22einstein+in+america%22&q=pretty+girl#search_anchor)
Attributed in posthumous publications
Variante: When you are courting a nice girl an hour seems like a second. When you sit on a red-hot cinder a second seems like an hour. That's relativity.