For example, consider a system of gas molecules in a box.

Because they have mass, the gas molecules will be affected by gravity.

As a result, light can pass through it without hitting the gas molecules very much.

Also, gas molecules tend to be rather small with only a few bonds in them.

This is because ideal gas molecules are so far apart that they don't interfere with each other at all.

This process can happen between a gas molecule and a metal rod.

They won't leak liquid, but gas molecules can go through.

Thus the mechanical energy of the blades is transferred to the gas molecules.

Yes, I realize that gas molecules typically move faster than the speed of sound.

They are closed and hollow, or have enclosed gas molecules.