Only whiskey produced in the United States can be called bourbon. Bourbon must be made of a grain mixture that is at least 51% corn. Straight bourbon must be aged in new, charred oak barrels for a minimum of two years. Bourbon must be distilled to no more than 160 proof, barreled at no higher than 125 proof, and bottled at no less than 80 proof. Bourbon can have nothing added to it except water.