A self-sealing valve is made of two pieces of plastic sealed on the edges with open ends. When air or helium is forced through the valve, the pressure opens the valve. When there is nothing to force open the valve, it closes and seals the air or helium inside the balloon. Below is a patent for a self-sealing valve. Others exist as well. If the wrong equipment is used to blow up a foil balloon, the self-sealing valve can expand or tear.