What is your favorite paradox?

A paradox is a statement or group of statements that leads to a contradiction or a situation which defies intuition. It can also be an apparent contradiction that actually expresses a non-dual truth. Typically, either the statements in question do not really imply the contradiction, the puzzling result is not really a contradiction, or the premises themselves are not all really true or cannot all be true together. The word paradox is often used interchangeably with contradiction. Often, mistakenly, it is used to describe situations that are ironic. The recognition of ambiguities, equivocations, and unstated assumptions underlying known paradoxes has led to significant advances in science, philosophy and mathematics. But many paradoxes do not yet have universally accepted resolutions. Sometimes the term paradox is used for situations that are merely surprising. The birthday paradox, for instance, is unexpected but perfectly logical:

The year has 365 days. In a room there are 57 students. I bet $100 that at least 2 of them have the same birthday. Do you take the bet?