bottle for example, they push on the sides of the bottle. this ‘push’ is what is known as pressure. if the size of the bottle and the amount of air in it stays the same, the pressure in the bottle will stay the same. but the pressure in the bottle can change. if the size of the bottle increases without allowing more air in, the pressure in the bottle goes down. this is because there are fewer particles inside than outside. if you then removed the lid, air would flow into the bottle. this would make the pressure on the inside the same as the outside.
getting fresh air into the 