The "angels' share" is the bit of the whiskey that evaporates through the pores of the oak barrels as the spirit ages, and while this sounds ho-hum, the phenomenon accounts for millions of liters of whiskey being lost every year. Everything from location and climate to wood type and airflow determine how much liquid a certain batch will lose.

The angels' share gets its name from the medieval Irish and Scots, who thought that the missing whiskey was an offering to the angels. Today, we have a better understanding of how evaporation impacts whiskeys like Kentucky bourbon, which is strong and high in alcohol because of Kentucky's high heat and humidity.

Heat causes whiskey to evaporate faster, speeding up aging, while humidity draws water out of the spirit before the alcohol, giving the bourbon a higher ABV. Alcohol aged in smaller casks also has more contact with the wood and evaporates faster, and some distillers give the "angels" a bigger share in order to make whiskey faster.