One thing that's astounding about Einstein's publications in 1905 is that they spanned
such a large breadth of physics: after illuminating the quantum nature of light by explaining
the photoelectric effect in March, in April he turned to something apparently more mundane:
particles suspended in fluids.
In particular, if you look at tiny particles in water, or dust motes in the air, you'll
see that they jitter about in a very odd and random way.
That is, it appears odd and random unless you believe that the air or water itself is
made up of even smaller particles called atoms or molecules, which just bounce off of each
other according to very simple rules.
This is called Brownian motion, despite the fact that botanist Brown wasn't the first
to discover it, and similarly Einstein wan't the first to describe it mathematically…
but he did draw the conclusion that the mathematical description of Brownian motion is evidence
for the existence of atoms even if you can't see them directly, and then he cleverly derived
how big atoms should be just based on how much the Brownian particles move.
That's like measuring the size of a penguin just by looking at how icebergs jiggle.
Talk about cold and calculating!