All matter,  whether gas, liquid, or solid,  is composed of very small pieces called atoms and molecules.  In air,  the molecules are very far apart,  so it is easy to press them closer together.  In water and sand,  the molecules are already very close together,  so it is very hard to push them any closer.

Remember that atoms and molecules are too small to see,  even with very powerful microscopes.  This cartoon is just to give you an idea of what happens when you compress a gas.
 Sand Water Air

Here are some things to think about:

What happens to the volume inside the syringe when you press down on it?

What happens to the number of molecules inside the syringe?

What happens to the density of the air when you push down on the syringe?

Now let's go back to the first question we asked you about density...

Is the density of a gas constant, or can you change it?  If it is not constant, how might you change it?  How about the density of a liquid, or a solid - is it constant?

Do you have any new ideas about density yet?  If so,  be sure to write them down.  At one time,  you may have learned a different definition for density - something like:
 density = mass/volume

Is this different than the definition you just learned for the density of gas?  (Hint:  the mass of anything is just how much matter it's made up of.  Usually,  when we talk about the weight of something - we are really just talking about its mass).

When you push down on the syringe,  the number of molecules of gas in the closed part of the syringe stay the same (well,  some might leak out,  since they are so tiny,  but generally the number of molecules stays the same...)  This means that the mass of the gas stays the same,  since the mass of the gas is just the mass of all of the molecules in there.  But, you make the volume smaller when you push on the syringe.  This makes the density of the gas much higher.

Here's something else to think about...

Besides getting squeezed together,  what happens to the molecules of gas when you press down on the syringe?

Before going on to the next section - be sure to write down your ideas about what happens to gas when you squeeze on it.