Oasis water bottles are one of the best water bottles on the market. They are made of hard plastic that is hard to chew, and special metal guards can be purchased separate, or included in the pack. They come in a variety of fun colors, and are sold for a reasonable price. sizes are 4 oz and 8 oz, both are a recommended size, depending on the number of gerbils per enclosure. They can be hung with a bent paper clip, or in the metal guards.