The thread post is what happened near the end of the book that I'm thinking of. There was a different species that lived along the humans, and were said to be creatures of earth (or somethign to that effect) and the main character was one of that species, but he loved sailing and going to high places, which was very odd for one of his species. The species looks like humans ( I think? ) but they live much longer than humans. The main character was about 100 or so, a teenager by the species' standards, and I remember him noting that it was odd that even though he was much much older than human's teenagers, that he still acted like them, or at least had the same maturity as them. Somehow he found a group of friends, and they set out to find an mysterious island that only appeared when all four elements came together. Either they set out to find it or there was another guy that tricked the main character to go onto the top of a mast on a ship in a storm or something, which summoned the island. The fountain of youth(i think?) is on that island, which is why the other guy wanted to find it. In order to get water from the fountain of youth that was on the island though, you need to give it water that it does not have. The other guy brought water from the bottom of a well as his gift. Or maybe they had to give water in order to get off the island, I don't know, but the group of kids that was with the main character had to give something to either get off the island or to receive the fountain of youth water to heal somebody. They didn't have anything, and the girl with them started to cry for the first time, and that was near the end of the book.
Sorry for the all over the place description, I don't remember any names. I read this in middle school, about 6 years ago.
Thank you!
