you breathe without even thinking about it.

Your body does it on its own, from the day you're born until the day you die.

You have muscles contract to bring oxygen, a gas,

into your lungs, which is then transferred by your bloodstream to every cell in your body.

Gases are strange.

We can't see them, but we know they're there because we can feel them.

What we experience as wind is really trillions and trillions of gas molecules slamming into your body.

And it feels good, right?

Science is based on observation.

Unfortunately, we cannot observe gases with our eyes -- they're too small.

We have to use our other senses to make observations and draw conclusions.

Observations are then compiled, and we create a model.

No, not that kind of model.

A model is a way scientists describe the properties of physical phenomena.

First, gases always move in a straight line.

We don't really have anything to demonstrate this with because gravity always pulls objects down.

So imagine a bullet fired from a gun, and that bullet goes on at a constant speed in a perfectly straight line.

That would be like a gas molecule.

Second, gases are so small, they occupy no volume on their own.

As a group they do, blow up any balloon and you can see how that volume changes.

But single gases have no volume compared to other forms of matter.

Rather than calculating such a small amount of matter, we just call it zero for simplicity.

Third, if gas molecules collide, and they do -- remember, these are assumptions --

their energy remains constant.

An easy way to demonstrate this is by dropping a soccer ball with a tennis ball balanced on top.

Because the soccer ball is bigger, it has more potential energy,

and the energy from the larger ball is transferred to the smaller tennis ball

and it flies away when that energy is transferred.

The total energy stays the same.

Gases work the same way.

If they collide, smaller particles will speed up, larger particles will slow down.

The total energy is constant.

Fourth, gases do not attract one another, and they don't like to touch.

But remember rule three. In reality, they do collide.

Finally, gases have energy that is proportional to the temperature.

The higher the temperature, the higher the energy the gases have.

The crazy thing is that at the same temperature, all gases have the same energy.

It doesn't depend on the type of gas, just the temperature that gas is at.

Keep in mind this is a model for the way gas particles behave, and based on our observations,

gases always move in straight lines.

They're so small, that they're not measurable on their own,

and they don't interact with one another.

But if they do bump into one another, that energy is transferred from one particle to another,

and the total amount never changes.

Temperature has a major effect, and in fact,

all gases at the same temperature have the same average energy.

Whew! I need to go catch my breath.