Watching dust jiggle under a microscope helped prove that atoms are real

In 1827, botanist Robert Brown noticed pollen grains jittering randomly in water for no obvious reason. In 1905 Einstein worked out the maths: invisible, fast-moving water molecules were bumping the grains. Jean Perrin's experiments confirmed Einstein's predictions in 1908, settling a century-long argument over whether atoms actually existed.

— Albert Einstein, Über die von der molekularkinetischen Theorie der Wärme geforderte Bewegung von in ruhenden Flüssigkeiten suspendierten Teilchen — Annalen der Physik, 1905