The tutor describes the phenomenon of Brownian motion.

Brownian motion is the unpredictable motion of particles suspended in a fluid. The movement is disorderly because it depends on collisions with fast-moving molecules of the surrounding fluid, which happen at random.

While attributed to Robert Brown (1827), the activity was observed earlier by Jan Ingenhousz (1785), and much earlier still by Lucretius (60BC). Specifically, Lucretius explained that dust particles seen moving in sunbeams are stimulated by collisions with invisible particles – atoms, he claimed. Einstein, in 1905, reached the same conclusion: Brownian motion, caused by collisions with unseen particles, proves that molecules and atoms exist.

Source:

wikipedia

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