Place the sugar in a medium bowl. Using your hands, rub the lemon and lime zest into the sugar until it's fragrant and fully incorporated.
Pour in 1 cup of boiling water and whisk until the sugar dissolves.
Strain the mixture through a fine sieve into another bowl to remove the zest, creating a lemon-lime syrup.
Let cool.
To serve, fill a glass with ice. Add about ¼ cup of the syrup (or more to taste), the juice of ½ lime, and top with sparkling water.
Stir gently and enjoy your homemade Sprite.
