system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: Why does diffusion happen?

Why do molecules have to spread out evenly? What is making that happen...
gpt
Take a bunch of blindfolded people and stick them in one corner of a fenced in backyard. Now tell them to wander around aimlessly. They're going to slowly spread out throughout the yard, occasionally bumping into each other and the fence, but eventually kinda spreading out evenly-ish.

Molecules are also just kinda moving around randomly bumping into each other, so eventually they'll start spreading out by random chance.