This is a favorite in Cuba. Ernest Hemingway was particularly fond of it.
Juice of 1/2 lime
1 teaspoon granulated sugar
2 or 3 fresh mint leaves and 1 sprig mint, for garnish
1 1/2 ounces white rum
Stir lime juice and sugar in a tall drink glass until the sugar dissolves. Add the mint leaves, crushed ice and rum, then stir. Top off the drink with club soda. Stir again, then garnish with the sprig of mint and serve.