[Johann Mahne]

On this forum i have learned, from the scientists, that gas molecules can spin.
What causes this spin?
Is it glancing collisions with other molecules, or photons hitting the molecules with off center impacts, or something else?
How is the spin measured?
I always thought that gas molecule collisions were elastic, but now i'm not sure. Is momentum conserved when they collide? What happens to the angular momentum if two oppposite spins collide?

[Soul Surfer]

The answer is yes generally. 

conservation of energy momentum and angular momentum always applies and it is quite possible for opposite states to cancel out

Rotation states like other atomic states are also quantised and can be detected by the spectrum of the thermal radiation from the gas.  This is usually quite well down into the infra red region of the spectrum