As any fule kno, Brownian motion is a random jiggling of small particles suspended in a fluid. The name derives from botanist Robert Brown who first observed it in pollen grains suspended in water. It appeared to be an example of perpetual motion until Einstein explained the phenomenon using kinetic theory. Basically he ascribed the motion to random fluctuations in the way that fluid molecules impinge physically on the much larger particles.
Over two thousand years ago, Lucretius observed Brownian motion in dust particles dancing in a shaft of sunlight, using it to prove the existence of atoms.
...look where the sun
Through some dark corner pours his brightest beams,
A thousand little bodies you will see,
Mix in the rays, and there forever fight
Arrayed in mimic troops, no pause they give
But meet and part again, nor ever cease.
From this you may conjecture of the germs [atoms]
What ‘tis for ever in the mighty void
To be tossed up and down. In some degree
Such small events may illustrate great things,
And give a clue to knowledge. So ‘tis well
That you should note these bodies how they rush
In the sun’s rays, because such rushes show
What secret hidden forces lie below.
Lucretius – On the Nature of Things.