The Moscow Mule's most famous component isn't an ingredient, it's the copper mug that traditionally holds the simple cocktail of vodka, ginger beer, and lime. It's the mug's burnished sheen that set the drink apart in the early days of the cocktail revival when vodka-based drinks were considered passé. The mug is the very reason the Moscow Mule exists in the first place. Where this began is up for debate, however.