This does not apply to all alcohol, only to spirits that are aged in barrels.
A barrel is (generally) charred on the inside and whiskey, rum, tequila, brandy, etc poured in for aging.
This spirit inside seeps into the wood. When it is warm (day, summer) the wood expands and more spirits seeps in. When it is cold (night, winter), the wood contracts and the spirit is pushed back into the barrel.
This is how the spirit gets its color and predominant flavors (notes of vanilla, caramel, butterscotch, chocolate, tobacco, etc).
The spirit is not pure alcohol; a large part of it is water. Water molecules are smaller than alcohol molecules, and so more water gets into the wood than alcohol. Some of this reaches all the way to the outer edge of the barrel and evaporates (the “angel’s share”). This has two effects: since more water evaporates than alcohol, the spirit inside the barrel goes up in alcohol/proof over time. That’s why you get labels and products offered at “cask strength”. But also, the older an aged spirit is, the less there is in the barrel when it comes time to bottle.
So, generally, well aged spirits have smoother and more complex flavors, higher alcohol content if bottled at cask strength (as opposed to having water added prior to bottling to lower the proof), take more time to produce and yield less product the longer they are aged, so they also must cost more.
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