Turtles All the Way Down

A renowned scientist was giving a talk on solar systems, galaxies, and the universe. When he got to the end of the lecture, an old lady stood up and interjected that everything he said was ridiculous, and the world is really just a flat sheet supported on the back of a giant tortoise. The scientist asked, "what is the tortoise standing on?" To which the woman replied, "it's turtles all the way down."