Most recent answer: 01/16/2018
When you compress a gas you're not compressing the individual atoms. For a gas, there are empty spaces between the atoms or molecules. Compressing the gas just means moving the atoms or molecules closer.
The gas atoms get their kinetic energy from all sorts of different sources. They bump into solid things, and the atoms in the solid things are rattling around themselves. They get hit by light, and that gives them a little momentum. Really, everything around us is jigggling with random thermal motions all the time. What's hard is not getting some of that energy, but getting rid of it. You can never cool things down to the point where all that energy is gone.
(published on 01/16/2018)