Combine the granulated sugar with 2 tablespoons of warm water in a small heatproof bowl or cup and stir until the sugar is fully dissolved to create a quick simple syrup. Let it cool to room temperature (about 2–3 minutes if water was only slightly warmed).

Mix flavorings

In a pitcher, add the lemon juice, lime juice, citric acid, lemon-lime extract, and the pinch of salt. Stir until the citric acid is mostly dissolved. Add the cooled simple syrup and stir to combine.

Combine with carbonated water

Add the chilled carbonated water to the pitcher slowly to preserve fizz. Stir gently once or twice with a long spoon — do not vigorously stir or shake. Taste and adjust: add up to 1 teaspoon more simple syrup if you want it sweeter, or up to 1/4 tsp more citric acid for brighter tartness. If using separate lemon or lime extract, adjust in 1/8 tsp increments.

Chill and settle

Let the mixture rest in the refrigerator for 5–10 minutes to allow flavors to meld while keeping carbonation. Serve immediately after chilling for best carbonation.

Optional Garnish & Serving

Fill a tall glass with ice cubes. Pour the prepared lemon-lime soda over the ice. Garnish with a lemon or lime wheel on the rim and a small mint sprig if desired. Serve cold.