Brownian Motion

This simple demonstration of Einstein's explanation for Brownian motion shows little particles batting about a more massive one, and what it would look like if you could see only the massive one through a microscope. Einstein showed that the overall visible motion, averaged over many observations, exactly matches what you would expect if the little particles were atoms or molecules. (This applet in Java 1.1 may not work in all browsers, and to "Reset" you may have to use your browser to "Refresh" or "Reload" the page.)

From http://galileo.phys.virginia.edu/classes/109N/more_stuff/Applets/brownian/applet.html