When you put small particles (pollen grains or dust for example) in a liquid (water for instance) you can see that the particles jiggle in all directions. You can observe that their motion is random. The pollen grains move irregularly in an unpredictable fashion. This is best seen using a microscope. Below is a short section from an article about this phenomenon, which is called "Brownian motion."
M.D. Haw, "Colloidal suspensions, Brownian motion, molecular reality: a short history", J. Phys.: Condens. Matter 14 (2002) 7769-7779.
"That the name of botanist Robert Brown should be singly associated with the phenomenon of 'Brownian motion' is perhaps somewhat fortuitous and not entirely justified. It seems that the ubiquitous, irregular motion of small grains suspended in fluids had been observed very soon after the appearance of the microscope. Generally, these early observations were interpreted as the motion of living creatures ('animalcules'). Brown's first contribution was to carry out, in the months of June, July and August, 1827 detailed observations of the motion of pollen grains suspended in water. In fact similar observations had been recorded in France, in 1824, by Adolphe Brongniart, while Brown's account did not appear until a year later. Both Brongniart and Brown at first presumed a 'living' origin of the motions, but Brown subsequently showed that inorganic grains also demonstrated an irregular motion in suspension. Observations were also made of gas bubbles in liquid inclusions trapped inside mineral crystals. That these irregular, apparently inexhaustible motions were observed in such a wide range of suspensions put paid to the idea of a 'vital' cause, demonstrating that the explanation, whatever it was, lay more in the realm of physics than biology."