1. Combine sugar with lemon and lime zest and rub them together until the sugar is evenly coated and fragrant.
2. Add boiling water and whisk until the sugar fully dissolves. Strain out the zest so the syrup is smooth, then let it cool.
3. Fill a glass with ice, add ¼ cup syrup, squeeze in ½ lime, and top with chilled sparkling water. Stir gently.
Fresher, cleaner, and free of preservatives or heavy corn syrup.
