When spirits like Scotch whisky, cognac, or bourbon are aged in wooden barrels, they don't stay at the same volume. Because wood is porous, a small percentage of the alcohol and water evaporates through the barrel walls over time. Distillers traditionally call this lost portion the 'Angel's Share.' In Scotland, this loss is about 2% of the barrel's volume per year, while in hotter climates like Kentucky (for bourbon), it can be much higher. This evaporation is crucial to the aging process, as it concentrates the flavors and allows the spirit to interact with the oxygen outside the barrel. Interestingly, if only water evaporates, the alcohol percentage increases; if only alcohol evaporates, it decreases. This delicate balance helps determine the final character and price of the aged spirit.