The pressure in the air around you is actually quite strong nearly 14.7 pounds per square inch! Normally, its effects go unnoticed. Your body is not only used to this external force, it depends on it: this is why its often uncomfortable on aircrafts where the ambient pressure is lower than at the earths surface. When you fill a bag with air, the pressure in the bag is equalized with the ambient atmosphere. The bag is comfortable with its surroundings, like you are at sea level. However, when you start to remove air from the container, a pressure difference is created between the inside of bag (higher pressure) and the outside of the bag (lower pressure). The force that was previously unnoticed is now very apparent the air on the inside of the bag will push outwards, inflating the bag. In truth, there is no more air in the bag than there was when the bag was deflated, its simply occupying more space, or volume. Return the bag to atmospheric pressure, and it will return to its original volume

The reverse is also true! Submerge the same bag in a deep pool, and the increased pressure from the water will squeeze the bag into a smaller volume. (Youve probably noticed that the water will try to squeeze your head to a smaller volume, too!) The relationship between pressure and volume can be quantified into an equation, known as the ideal gas law.