Brownian motion. Gas diffusion.
Sprinkle some dust on water and watch them move. Many people have studied this Brownian motion. Even Einstein.
Have 2 glass jars connected to each other but separated by a retractable door. One jar has coloured gas while the other is of normal air. Retract the door and watch the mixing occur. You can see the coloured gas filling up the 2 glass jars.
Fun fact: It is possible but statistically improbable for the coloured gas to be back into their original jar despite removing the door and moving at random.