Most balloons are made out of latex which is a white milky natural liquid that comes out of some gashed trees. Once it is collected it is shipped to factories and turned into rubber that they use to make balloons. To inflate a balloon you exhale air into it. After it is inflated you twist the end and tie a knot. The reason the air is trapped in the balloon is because the molecules are too large to pass through any holes in the balloon.

NJM Example of how a balloon traps air..jpg
Example of How the Balloon traps airAnd how it deflates.

Latex is a white milky liquid that come out of trees to heal their wounds. Rubber from the latex of the rubber tree reacts badly to extremes of temperature. Charles Goodyear, an American inventor, simultaneously with Thomas Hancock in England, heated rubber with sulfur and found that this rubber remained flexible over a range of tempuratures.

NJM latex collection.jpg
Example of How People collect Latex

Air Pressure
Air pressure, technically, is a measurement of the amount of collisions against a surface at any time. In the case of balloon, it's supposed to measure how many particles at any in any given time space collide with the wall of the balloon and bounce off. However, since this is near impossible to measure, air pressure seems to be easier described as density. The similarity comes from the idea that when there are more molecules in the same space, more of them will be heading towards a collision course with the wall.

Latex: A type of rubber that oozes out of trees to heal gashes.
Air Pressure: the measurement of how many times contained molecules hit a surface in a certain amount of time.

Newmark, Ann. Chemistry. London Dorling Kindersley, 1993. Print.
Brain, Marshall. How Stuff Works. New York : Hungry Minds. 2001
Latex. Wikipedia. 12 September 2012. September 14, 2012.