Diffusion is the natural spreading out of something from where there's a lot of it to where there's less of it.

Drop a dot of food coloring into a glass of water and watch what happens: without any stirring, the color gradually spreads until the whole glass is tinted. That's diffusion. The colored molecules move from the concentrated drop into the surrounding clear water, mixing evenly throughout.

Diffusion happens everywhere. When you smell cookies baking in the kitchen while sitting in your bedroom, that's diffusion: odor molecules spread through the air from the high concentration near the oven to the lower concentration in other rooms. When you open a bottle of perfume, the scent molecules diffuse outward. Even in your body, oxygen diffuses from your lungs into your bloodstream, moving from where there's more oxygen (in the air you breathe) to where there's less (in your blood).

The process requires no stirring, mixing, or outside force. Molecules naturally move around randomly, and over time, this random movement causes them to spread out evenly. Scientists call this moving down a concentration gradient, from areas of high concentration to low concentration, until everything is distributed uniformly.