Why can liquid particles diffuse through a liquid?

Diffusion in a liquid occurs because of the constant, random motion of its constituent particles—such as molecules, atoms, or ions. According to the kinetic molecular theory, this motion is a manifestation of thermal energy; the greater the thermal energy, the faster the particles move.12

While particles in a liquid are held together by forces of attraction that give the substance a high density, they are not locked into the rigid, ordered structure of a solid. Instead, liquid particles continue to move about within the liquid state. This random movement allows for diffusion, a process characterized by the net flow of matter from a region of high concentration to a region of low concentration. Although diffusion occurs more slowly in liquids than in gases—due to the higher density and closer proximity of particles in the liquid state—the underlying mechanism of random particle motion remains the driving force for this transport.123