On the right, the jiggly path of a tiny particle observed through a microscope. On the left, Einstein's explanation: buffeting by (much tinier) molecules.
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Biologist Robert Brown, studying the sex life of plants, noticed in 1827 that very tiny (a few microns size) pollen grains could be seen jiggling around under the microscope. At first he thought this was a sign of life, but then checked with equally tiny particles of rock, and saw the very same motion. After a few decades, it was realized that this must be caused by molecules bouncing off the particle at random. Then Einstein figured out that the smaller the molecules (and therefore more of them for a given density liquid) the less the particle would be jiggling, and with some clever math, Einstein was able to find the size of the molecules from measurements of the particle's path--one of the first reliable measurements of molecular size. All the details are in my lecture here.
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