The question, "What came first, the chicken or the egg?", was a paradox, as chickens were hatched from eggs, which were laid by chickens. So the existence of one ensured the existence of the other, but which started the process was a mystery.
The Seventh Doctor entertained the Gods of Ragnarok with a magic trick where he drew eggs from his mouth, after first asking, "How did life begin? Was it with the chicken, or..." The Gods seemed unfamiliar with this concept. (TV: The Greatest Show in the Galaxy)