For this unit to work properly, you need the older 12-16 fl ounce plastic pop bottle with the longer screw top opening. Some of the bottles made today have a shorter screw top opening which allows the water to flow out and run over the drink cup because it does not fit far enough into the drinker cup to create an air lock. The older plastic pop bottle with a screw top opening goes down in the drink cup far enough to create an air lock. Also you need to remove the remaining ring from the tamper proof cap. Simply take a sharp pair of scissors and cut the plastic ring off the screw top opening of the pop bottle. Waterers will fit through the 1″ wide wire on the cage and the provided spring will hold the bottle against the wire.