Ship of Theseus

The Ship of Theseus is a thought experiment that's been around for centuries. In it, a ship is made from many parts, and over time, each part is replaced until there is nothing left from the original. The question is then “Is it still the same ship?”

To explain it in a way a 5 year old might understand, you could say that it's like a toy made out of Legos. Over time, you may use the original pieces to build something else or replace them with new ones. After enough pieces have been changed, you may start to wonder, "Is it still the same toy?"