whatOne heavy grain -- thirty times the mass of anything else in the box -- in a bath of hundreds of light particles drawn so faintly they are almost rumor. No synthetic noise anywhere: every jiggle is a real elastic collision, energy conserved to rounding error. The grain inks its path as it wanders: Perrin's drawing, redrawn forever.

whyThe claim: the jitter has a law. Displacement looks lawless, but its average grows exactly as the square root of time. The panel measures it from the one trajectory itself -- every window of its own history is a fresh experiment: at short lags the grain remembers its velocity and mean-square displacement climbs like t-squared (ballistic); past the crossover the bath beats the memory out of it and the slope settles onto t -- Einstein's 1905 line, drawn as a guide anchored at a single measured point. Raise the temperature and the dance quickens, D climbs, the law holds.

howThe elastic-disk kernel with unequal masses; mean-square displacement over log-spaced lags from overlapping windows; slopes fitted log-log; the guide is 4*D*tau with D taken from one reference lag. The slider rescales bath speeds and restarts the statistics. One integer seeds the bath.