A scientist once gave a public lecture describing how the Earth orbits around the sun and how the sun, in turn, orbits around the center of a collection of stars called our galaxy.
At the end of the lecture, a little old lady at the back of the room got up and said: "What you have told us is rubbish. The world is really a flat plate supported on the back of a giant tortoise."
The scientist gave a superior smile before replying, "What is the tortoise standing on?''
"You're very clever, young man, very clever," said the old lady, "but it's turtles all the way down!"