The subtle point is the following: there are an enormous number of molecules of air. On average, as many hit the granule from the left as hit it from the right. If the air’s molecules were infinitely small and infinitely numerous, the effect of the collisions from right and from left would balance, and thus cancel out at each instant, and the granule would not move. But the finite size of the molecules, the fact that these are present in finite rather than infinite number, causes there to be fluctuations (this is the key word): that is to say, the collisions never balance out exactly; they
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