Which ingredient is added as a float in the Rum Runner?

In a Rum Runner, grenadine is added as a float to create a visually appealing layered effect in the drink. Floating grenadine on top of the cocktail not only adds a pop of color but also enhances the flavor profile with its sweet and fruity notes, balancing the rum and other ingredients typically used in this tropical cocktail. This technique of floating grenadine allows it to mix gradually with the other ingredients when consumed, offering a delightful taste experience.

Other ingredients like soda, simple syrup, or bitters do not serve the same purpose in the context of a Rum Runner. Soda is generally used for carbonation, simple syrup is used for sweetness but is mixed in, and bitters are typically meant to be integrated throughout for flavor complexity rather than floated on top. Thus, the use of grenadine as a float aligns perfectly with the characteristics and presentation of the Rum Runner.