Combine the granulated sugar and 1/2 cup filtered cold water in a small saucepan. Add the lemon and lime zests. Heat over medium, stirring constantly until the sugar dissolves and the mixture just comes to a simmer (do not boil vigorously). Once the sugar is fully dissolved, remove from heat and let steep for 5 minutes to extract the citrus oils from the zests.

Strain the syrup through a fine-mesh sieve into a small bowl to remove the zests. Stir in the fresh lemon and lime juices. Taste and adjust: if you want it sweeter, add up to 1 tablespoon more sugar and stir until dissolved (you can warm briefly off heat). Let the syrup cool to room temperature (about 10–15 minutes).

Assemble Sprite-style Soda

Chill glass and ice (optional)

If using ice and chilled glassware, fill the serving glass with ice now and set aside. For a very crisp result, use chilled club soda and a cold glass.

Combine soda and syrup

In the serving glass or a pitcher, add 2 tablespoons of the cooled citrus syrup (about 30 ml) per 8 oz (1 cup) of club soda. For this recipe's 2 cups of club soda, add 4 tablespoons (1/4 cup) of syrup. Stir gently to combine so as not to flatten the carbonation. Taste and adjust: add more syrup for sweeter, more citrus-forward soda or more club soda for milder flavor.

Garnish the drink with a lemon wheel, lime wheel, and a sprig of mint if desired. Serve immediately while fizzy.