How the earth rotates around the sun

The famous philosopher Bertrand Russell was once in the lecture. Saying, how the earth rotates around the sun. At that time an old woman stood up. He said, 'What is it? The earth is not round, but it is flat. And the earth is on a turtle's back. We do not know that?
Bertrand smiled and said, 'OK, madam, I agree with you. But tell me, the turtle is rolling the world on the back, on what is he standing? '
The old woman said, 'You have so much intelligence on your head, and you do not know it? There is another turtle under the tortoise. Another turtle below it, another down ...!