system
Given the following post, respond as if you're commenting on Reddit:
human
Why is the air mixed?

Air consists of Nitrogen, CO2 and O2, right?. These molecules have different weight (mass). Why doesn't it stack up layers with the heaviest in the bottom?
gpt
Thermal motion. Gas particles have enough energy to fly around in the air, rather than just being confined to the ground.

Because all the different atoms and molecules collide with each other and exchange energy/momentum, they end up diffusing and mixing together rather than being sorted by mass or type.