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the random movement of molecules leading to a “net movement” from areas of high concentration to areas of low concentration (down a concentration gradient). Think about if you have a bunch of people in one area surrounded by open space and those people start moving about randomly – they’re naturally going to get more spread out because there’s all that open space to go to – most ways they randomly go will have the effect of spreading them out more. And if they try to go to a crowded space they’ll get knocked off course. End result is that net movement from high concentration to low concentration and eventually an even distribution of the molecules (but still molecules moving around, just balanced out).