Once Albert Einstein's wife was given a tour of the Mount Wilson Observatory in California. There she viewed the giant telescope, at the time among the largest in the world and even today a marvel of mechanical precision.
As part of her tour the leading astronomer took time to tell her how the complex machinery worked.
"One of the principal functions of all this sophisticated machinery," he explained, "is to find out the extent and shape of the universe."
"Oh," was her ingenuous response, "My husband does that on the back of an old envelope."