Add the lime juice and ginger beer to the glass. (Stir gently, if you like).
Top with the rum and garnish with a lime wheel.
Notes
Chill the glasses before serving. The ice will melt slower, so the drink does not dilute.
Garnish with a lime wheel, twist of zest, or a wedge of lime.
To float or not to float? The black rum floating on top of the ginger beer makes an excellent presentation, but it can also be stirred together after being topped with ginger beer.