If you replace every plank of a ship, is it still the same ship?

It depends on what you mean by "same", and that is exactly why philosophers have argued about it for over two thousand years. The Ship of Theseus is a thought experiment about whether an object stays the same object once every one of its original parts has been swapped for new ones.

The story comes to us from Plutarch. Every year, the Athenians sailed the ship of their legendary king Theseus to the island of Delos to honour Apollo, and as its timbers rotted they replaced them. Eventually people asked: if not a single original plank remains, is it still Theseus's ship? And if it isn't, at which plank did it stop being?

In the 17th century Thomas Hobbes made it worse. Imagine a custodian quietly keeping every discarded plank and rebuilding a second ship from them. Now there are two candidates, one that kept the form and one that kept the matter. Which is the original?

The puzzle is not just ancient trivia. The USS Constitution, a frigate from 1797, may keep only ten to twenty percent of its original wood, and its museum uses it as an example of the paradox. Japan's Ise Grand Shrine is rebuilt with new wood every twenty years. Philosophers of mind even use the ship to think about what keeps you being you.