The classic copper cup that Moscow Mules are served in gives them a distinct and stylish look. However, there's more to the reasoning for serving this drink in a copper mug than the aesthetics. Copper is good at holding temperature. So, when you pour the iced cocktail into a copper Moscow Mule mug, the cup takes on the temperature of the ice-cold liquid. As a result, the classic vodka-based cocktail stays chilled longer than it would served in classic cocktail glasses.