Although air is invisible, it still takes up space and is made of molecules. This is evident when the balloon is placed inside the bottle and you try to inflate it. It’s nearly impossible to add any amount of air! When the bottle with the hole is used, however, inflating the balloon is nearly effortless. The air inside the bottle is able to escape, freeing up space for the balloon to now inflate. If you then plug the hole on the outside of the bottle, the balloon will remain inflated. How does this happen? What keeps this air in place? This is a consequence of the air pressure being lowered inside the bottle when its hole is plugged. The high pressure air inside the balloon is pulled toward the low pressure area inside the bottle. When you add water inside the balloon then unplug the bottle, watch out! Unplugging the bottle will release low pressure’s hold on the higher pressure air inside the balloon and allow outside air to enter the bottle once again. Not only will the balloon collapse, the water inside of it will be propelled by the force of the air.