Combine granulated sugar and 2 tablespoons cold water in a small saucepan. Warm over low heat, stirring constantly, until the sugar dissolves completely (about 2–3 minutes). Do not boil. Remove from heat.

Stir the grated lemon and lime zests into the warm syrup and let steep for 5 minutes to extract oils. If using citrus extract, stir it in after the syrup is off the heat.

Strain the syrup through a fine mesh sieve into a bowl to remove zest solids, pressing to extract as much liquid as possible. Discard solids. Allow syrup to cool to room temperature (about 10–15 minutes) or chill briefly.

In a pitcher, combine the chilled carbonated water with the fresh lemon juice and fresh lime juice. Gently stir once or twice to mix — avoid vigorous stirring to retain carbonation.

Add the cooled citrus syrup to the carbonated water mixture and stir gently to incorporate. Taste and adjust: add up to 1 tablespoon more sugar syrup for sweetness or a few drops more lemon/lime juice for brightness. If you used the optional extract, add no more than indicated to avoid overpowering.

To Serve

Fill two tall glasses with ice cubes. Pour the homemade Sprite evenly into the glasses over the ice.

Garnish each glass with lemon and lime slices and a sprig of mint if desired. Serve immediately while fizzy.