Hydrapak Splash Guard Insert provides smaller drinking aperture to block splashes while on the go and a larger fill-through drain to add water liquid without removing Opening is sized for both kids and adults Food-grade silicone rubber material won't absorb or retain odors and tastes; can be sterilized by boiling and is dishwasher safe Flexible design is sized to fit wide-mouth bottles from most manufacturers, including Nalgene, Cyclone, Bison and GSI