All molecules are in constant motion unless at absolute zero. The motion is random, with molecules moving in straight lines until they encounter other molecules. The net result of this process is diffusion: the movement of a substance from an area where it is in high concentration to an area where it is in low concentration. When drops of dye are added to a beaker of water, the drops are areas of high concentration of dye molecules. They will eventually be evenly distributed among the water molecules, and although all the molecules continue to be in motion, no further change will be observed.