Using a microplane, zest roughly 4 limes and 3 lemons, then juice them. Set the juice aside.
To a medium-sized bowl, add the granulated sugar and lemon and lime zest. Use your fingers to massage the sugar into the zest, until there are no white streaks remaining in the sugar.
To a small saucepan over medium-low heat, add the zest and sugar mixture and the juice. Heat to a gentle simmer, stirring occasionally until all of the sugar has dissolved, then turn off the heat and let the zest steep in the syrup for at least 20 minutes, ideally until fully cooled.
Once cooled, strain the syrup through a fine mesh sieve to remove the zest. Transfer the syrup to a mason jar or other storage container.
To make your Sprite, add anywhere from 2-4 tablespoons of syrup to an ice-filled glass and then top off with sparkling water. Stir gently and enjoy!
