The teeth on hamsters, gerbils and rabbits grow like a human's fingernails.  They need to chew wood or something hard on a regular basis to keep the teeth filed down.  In an extreme case, if they are not kept filed down, the teeth can actually grow into the jaw of the animal, preventing it from eating and drinking!  It is wise to check the cage every day to make sure they have something to chew.  Also, check to make sure they have not chewed the edges of the plastic water bottle, causing it to leak.