This foam donut acts as a bumper to protect the front of your PET bottle against landings on hard surfaces. Part of the FUN in flying your Quest water rocket is experimenting with different amounts water and air pressure. Sometimes, if you add too much water, it shifts the center of gravity too far back and your rocket will fly unstable. The hole in the bumper can be used to easily add nose weight to your rocket such as clay. How about a game of Splat Landing??? Just add a regular size paintball in the hole. Makes a mess at the landing point - but boy, do kids love it!!! Safety Note: remember to always check your PET bottle for any damage before each flight.