A wooden cask, traditionally made from white oak staves by a cooper, used to age and store spirits such as whiskey and rum, as well as wine. During aging, the liquid slowly extracts tannins, vanillin, and other compounds from the wood, softening and adding complexity to its flavor; charring the barrel's interior, a common practice, adds notes of vanilla, caramel, and smoke. Bourbon and American whiskey are required by law to be aged in new, charred oak barrels, while other spirits, such as rum and tequila, are often aged in barrels previously used for something else. Oak barrels are occasionally used to age foods such as vinegar, cheese, or smoked meats.