Can a barber be a barber if he does not shave himself?
Or does he not count as a barber?
Zeno's DilemmaYou travel back in time and kill your own grandfather, preventing your own birth.
But wait, if you never existed, who killed the grandfather?
Krasner's KettleHow many grains of sand are on the beach? Is it more than 5, less than 7?
Or are there no grains at all?
Schrodinger's Cat