Diffusion: a crowd spreads out, though no one is heading anywhere
240 coloured particles start in the left half of a glass tank. Every one of them only jiggles at random. Pull the divider out and watch what that randomness does to the crowd, then keep watching after it levels off.
Share of particles in each half over timegreen band = within 7% of equal
Why the crowd spreads
No particle is aimed at the empty side. Each one is knocked about at random. But when one half holds all the particles, far more of them are in a position to wander across than in the empty half, so the count drifts toward the emptier side. Spreading is a statistic of many random paths.
Why it stops changing, not moving
When the halves hold about the same number, crossings left→right and right→left happen about equally often, so there is no net flow. Individual particles keep moving randomly. Follow the ringed one: its path stays erratic. The counts only wobble a little because 240 is a finite number.
What temperature changes
Warmer means larger random kicks, so the same spreading happens sooner. The final 50/50 split does not change. This is a simplified model: particles do not collide with each other, and their speed scales with the square root of temperature.