Robert Brown saw random, erratic movements of the pollen grains when he was looking at them under his microscope. This movement is now known as Brownian motion.

He observed that the pollen grains, even though they were inanimate, were constantly moving in a seemingly chaotic and unpredictable way. This was a significant discovery because it provided evidence for the existence of atoms and molecules, which were not yet widely accepted at the time.

Brown's observation led to further research on the nature of matter and helped lay the foundation for the development of statistical mechanics.