a valve device that is incorporated into the cap used on plastic bottles , which allow the bottles to receive the injection of gaseous pressure so that the walls of the plastic bottles become firm and rigid , which maximizes the bottles floatation potential . this allow the bottles to be handled and stored easily . these bottles have uses for many types of floatation needs , such as buoys , rafts , etc . the bottles are particularly useful as floatation devices in the sport of fishing , with the addition of fishing lines and hooks .