There are many versions of the “turtle” story. Here is one of the best known:

“William James, father of American psychology, tells of meeting an old lady who told him the Earth rested on the back of a huge turtle. "But, my dear lady", Professor James asked, as politely as possible, "what holds up the turtle?" "Ah", she said, "that's easy. He is standing on the back of another turtle." "Oh, I see", said Professor James, still being polite. "But would you be so good as to tell me what holds up the second turtle?" "It's no use, Professor", said the old lady, realizing he was trying to lead her into a logical trap. "It's turtles-turtles-turtles, all the way!”

— from Wilson, R.A. (1983, 1997) Prometheus Rising. Phoenix, AZ: New Falcon Publishers, 1983.