Chinchillas chew by nature, as momoftara&nell said, their teeth grow continuously, and they need to chew to keep their teeth trim. I keep chinchilla chew blocks in all of the cages, as well, I use wooden shelves and ledges for them to chew on. If after that, the chinchillas still chew the water bottles, then I usually switch to glass bottles or buy metal bottle guards to put between the bottle and the cage.