What are your favorite PARADOXES?

Question:

What are your favorite PARADOXES?

You know, those much loved (and hated) subject that you can fustrate yourself over for hours, often without result?

Best Answer - Chosen by Asker: i can think of two. the first comes from a book called 'Windjammer Run'. It is simply the phrase 'I always lie'.
the second one is how if it was possible to go back in time to change history, once you had made the change you would never have a reason to go back in time so the change would never be made so you would go back in time so youd have no reason to go back in time etc.
i get quite annoyed when tv shows show people travelling back in time. it defeats narrative structure, people!