Place the saucepan over medium heat and bring the mixture just to a simmer, stirring occasionally, until all sugar is fully dissolved (about 3–4 minutes). Do not boil vigorously.

Add citrus zest and juices

Remove the pan from heat. Stir in the finely grated lemon zest (1 tsp), lime zest (1/2 tsp), 2 tablespoons fresh lime juice and 1 tablespoon fresh lemon juice. Let the syrup steep for 10 minutes so the citrus oils infuse.

Strain and chill

Strain the syrup through a fine mesh sieve into a clean bowl or jar, pressing the zest to extract flavor. Discard solids. Let the syrup cool to room temperature, then chill in the refrigerator for at least 15–20 minutes. The syrup can be stored refrigerated up to 1 week.

Assemble the Sprite

Prepare glasses

Fill two tall serving glasses with ice (about 1 cup ice per glass). Add a lemon or lime wheel to each glass if desired.

Mix syrup and carbonated water

For each glass, add 2 tablespoons of the chilled citrus syrup. Pour 2 cups chilled carbonated water over the syrup (or 1.75 cups carbonated water + 0.25 cup sparkling mineral water for stronger fizz). Stir gently once to combine, taking care not to flatten the carbonation.

Adjust sweetness and tartness

Taste and adjust: add additional syrup (1–2 teaspoons) for more sweetness, or a squeeze of fresh lemon or lime for more tartness. Stir gently to incorporate.

Finish and garnish

Garnish each glass with a lemon or lime wheel and a sprig of fresh mint if using. Serve immediately while fizzy.