As whisky ceases aging after it is bottled, its complexion can change as there is no such thing as a positive seal. Some seals ( by luck ) are better than others ( e. g., two of the same cork in identical bottles ). By not having a positive seal, whisky over time becomes oxidized thus changing the character of the contents. Oxidation is the enemy as alcohol evaporates quicker than water. R. J. S. McDowall sums it perfectly. Professor McDowall states that this process changes the whisky in such a way that it "brandifies". But this process takes many years to happen. If you would like to test this theory but not wait years to find out the affects, pour a dram of Scotch in a tulip shaped glass, and let it sit overnight uncovered on the kitchen counter. When you wake the next morning, pour an identical dram from the same bottle in the same type of glass and then nose the two side-by-side. You will see a noticeable difference when nosed side-by-side. Then taste the two. Taste the freshly poured dram and swirl it around in your mouth and slowly swallow. Cleanse the palate with a piece of dark chocolate chased by some spring water. Then taste the dram that sat overnight. You will find that this dram to be more gentle as most of the harsher alcohols evaporated overnight. This is a "quick and dirty" way to experiment what oxidation does to a dram.