The chemical structure of the whisky will change in your glass over time as it breathes. The alcohol and the more volatile molecules will evapourate at a faster rate than the other molecules. Therefore, the character of your whisky will become different over the course of an hour. This is especially true for our single cask whiskies because they have higher alcohol levels to start with. It can be fun to leave a dram to sit for a few hours before comparing it to the same whisky poured fresh from the bottle.