Update: Looking over the old answer, which just popped up as the question of the day, it doesn't quite answer the question. After all, pressure becomes equal even when the environment consists of different materials, such as a region high in argon gas and one without any argon gas, or the air just above some shallow water and the region in the shallow water. The reason is simple. Say that the pressure in two different regions were different. The material in between would be pushed harder from one side than the other. It would move toward the low-pressure region, building up the density there. This process goes on (maybe with some oscillation) until the pressures becomes equal, even when the materials (water, air...) are different. Mike W.
Thanks for your question...it's a good one.
To answer your question, let's first think about what pressure really means by looking at some examples:
The pressure inside a balloon is what gives it it's round shape. You know that if there were no air in a balloon, it would shrivel up. The air in the balloon is causing a force that pushes the sides of the balloon outward. How does this work? Every breath of air is made up of billions and billions of molecules. When you blow up a balloon, you are actually filling it with molecules. Molecules are very small and light and move very fast, causing them to bounce off the walls of the balloon all the time. Every time a molecule bounces off the inside wall of a balloon, it gives it a little push in the outward direction, which is what keeps the balloon from shrinking. The force of the molecules hitting the inside walls of the balloon is called "pressure". The more air we put in a balloon, the mor e molecules there are to bounce off the walls, and the higher the pressure will be.
Since the molecules inside a balloon are moving in every possible direction, pushing up as much as down as much as left as much as right, the pressure is equal everywhere inside the balloon. The same is true with other examples of pressure. For example, when you swim to the bottom of a pool you can feel the water pressure pushing inward on your body, maybe even making your ears hurt a bit. Water, of course, is made of molecules just like air is, and when you are under water these molecules are bouncing off you body, pushing inward in the same way that the air in a balloon pushed outward. The water molecules surround your body, bouncing and pushing equally in all directions, so the pressure is the same on every part of your skin.
These are just two examples, but the same idea is true in general. The reason the pressure is "spread evenly" as you put it is that the medium that causes the pressure (air in the balloon, water in the pool) evenly covers whatever it is acting upon and pushes equally in all directions.
(PS: Notice that the pressure your body feels in a pool is lower at the surface of the pool and increases as you dive deeper. This i s due to the weight of the water, and is really not what you were asking about so we won't discuss it here. If you are interested in finding out more about this, you can ask us another question.)
(published on 10/22/2007)