This joke is from the opening paragraph of Stephen Hawkings book,
A Brief History of Time.

A well-known scientist (some say it was Bertrand Russell) once gave a
public lecture on astronomy. He described how the earth orbits around
the sun and how the sun, in turn, orbits around the center of a vast
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?

Youre very clever, young man, very clever, said the old lady.
But its turtles all the way down!