Molecular diffusion

Molecular diffusion is like a game of tag. In this game all the molecules are playing tag, but instead of tagging each other they're spreading out into empty spaces. When there's an area with more molecules, they'll spread out into the area with fewer molecules and vice versa. This is why sometimes when you open a bottle of something smelly, you can smell it really quickly – the molecules are spreading out and traveling to all the other molecules in the air.