Wines can sometimes be improved just before serving by a period of aeration or “breathing.” Such a treatment allows volatile substances in the wine to escape into the air, and it allows oxygen from the air to enter the wine, where it reacts with volatile and other molecules and changes the wine’s aroma. No significant aeration occurs when a wine is simply uncorked and left to sit in the open bottle. The most effective way to aerate a wine is to pour it, and into a broad, shallow decanter that continues to expose a large surface area to the air. Aeration can improve a wine’s aroma by accelerating the escape of some off-odors (for example, excess sulfur dioxide in some white wines), and by providing a kind of accelerated aging to young, undeveloped red wines. But it allows desirable aromas to escape as well, and can undo the complexity of a mature wine that has developed slowly over years in the bottle.