The measurement Einstein deemed impossible

Brownian motion, the seemingly random wiggle-waggle of particles suspended in a liquid or gas, was first systematically studied by Robert Brown in 1827 and described in the Philosophical Magazine the next year (volume 4, page 161). When Brown used a microscope to look at particles from pollen grains immersed in water, he “observed many of them very evidently in motion.” It looked like the particles were alive, so vigorously did they move.