The secret of the “Balloon in the bottle” trick

An awesome experiment that will make you a superstar at any party!
Equipment: rectangular plastic bottle, balloon.
Insert a balloon into a rectangular plastic bottle and secure it around the bottle neck. We recommend using a rectangular bottle because it is more stable and will hold its shape better. Try to inflate the balloon – you can’t. Just make an inconspicuous hole in the bottle and try again – the balloon should inflate!
Instinctively, we’d probably say the bottle is empty, but it isn’t – it’s filled with air. In inserting a balloon into the bottle, we create a closed system, the pressure in which is equal to atmospheric. In this closed system, our lungs can’t apply the pressure needed to compress the air inside the bottle enough to free the space for the balloon. The trick is simply to make an inconspicuous hole in the bottom of the bottle. This allows the excess air to leave the bottle, which lets you inflate the balloon. The hole must then be plugged with a finger or taped to prevent the balloon from deflating.
The balloon deflates slightly at the moment we close the hole, marginally decreasing the air pressure in the bottle. This difference between the atmospheric pressure inside the balloon and the reduced pressure in the bottle keeps the balloon inflated. The balloon’s deflation is practically imperceptible – only a miniscule change in the volume of free space in the bottle is necessary for the balloon to stay inflated. When we let air in through the tiny hole, the pressure in the bottle gradually equalizes with atmospheric pressure, and the balloon deflates due to its own elasticity.
Safety precautions: none.