To expand on that a bit... back in the "old days" people who made and sold alcohol had to prove that there was as much alcohol as they claimed.
A mixture of ethanol and water must be at least 38% (preferably 40%) in order to ignite and sustain a flame.
So the test was to put a little gunpowder into a dish and pour some of the whiskey or whatever on it and light it on fire.
If it burned and ignited the powder, it was "proof" that the whiskey was 100% "good".
The "100% proof" was actually about 40% alcohol, so to be useful in "modern times", the 40 was bumped up to 50 so that "100 proof" meant "50% alcohol" and of course "200 proof" means pure alcohol.
Note that it's impossible to produce pure 200 proof ethanol via distillation. At a concentration of 95.6 percent alcohol and 4.4 percent water, a mixture called an "azeotrope" is formed (i.e. both constituents have the exact same boiling point).
This is similar to "eutectic" metal alloys where 2 or more metals are mixed together to lower the melting point. A good example is solder. A 63/37 ratio of lead to tin melts at a lower temperature than either pure lead or pure tin.
Anyway, the azeotropic ethanol of 95.6 percent can only be purified to absolute 100 percent (200 proof) by chemically absorbing the rest of the water typically by the use of potassium hydroxide or sulfuric acid, then distilling off the pure ethanol in a closed system (so that it cannot absorb atmospheric moisture and become less than 100% again). Alcohol is very hygroscopic (it absorbs water).
Fun facts for today.
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