Ship of Theseus Paradox

The greek historian Plutarch wrote in the year 75 that the ship of the legendary Theseus was preserved by the people of Athens for hundreds of years. Whenever some part got too worn out they would replace it. Eventually, it was not clear if any part of the ship was still original  And if every part had been replaced, is it still the same ship?

And what if you found and gathered all the original, replaced and discarded parts and reassembled them into a boat - which of the two would be the true Ship of Theseus? 

Similarly, we know our own bodies are always replacing parts. How much of you is the same as the you that existed when you were a small child? Enough to say that you are the same person?

The answer, of course, is that we are the same person. And most would say the Ship of Theseus is the same ship, but it's instructive to think about why. It seems a thing does not derive it's thingness from it's parts alone. It's the relationship of those parts that is important, and the persistence (and evolution) of that relationship through time.