What is a typical requirement for a spirit to be classified as bourbon?

Bourbon must be made with at least 51% corn to meet the legal definition established by the U.S. government. Corn provides the distinctive sweetness and flavor profile that characterizes bourbon. This high percentage of corn is crucial as it differentiates bourbon from other types of whiskey, such as rye. The other requirements for bourbon, which include being aged in new charred oak barrels and being produced in the United States, do not specifically relate to the grain composition, making the grain content a foundational aspect of its classification. Therefore, the correct answer points to the essential ingredient mix that defines bourbon.