How can random motion produce net diffusion?

A drop of dissolved color can spread even when no one stirs it. Molecules are already moving and colliding. Individual particles do not choose to escape a crowded region, but many random movements together can produce a predictable net spreading.

Concentration compares an amount of substance with the space containing it. In the pictured equal volumes, twelve symbols begin on the left and four on the right. More particles are initially available to cross from left to right, even though individual crossings can occur in either direction.

The selected counting example sends six rightward and two leftward. Subtracting the opposite transfers gives four net rightward. The left side ends with 12 − 6 + 2 = 8; the right ends with 4 − 2 + 6 = 8. All sixteen symbols are still present. The dashed boundary marks regions in one connected space, not a wall blocking movement.

This exact exchange is a teaching record, not a simulation predicting which molecules cross or how long diffusion takes. Actual crossings fluctuate. At equal concentrations, opposite transfers balance on average while individual motion continues. A small snapshot need not always show exactly eight on each side.

Diffusion combines restless local motion with an overall trend toward mixing. Equal concentrations mean no expected net transfer between the regions, not that the particles stop. The quiet appearance of a mixed liquid conceals continuing microscopic motion.