Prep: 15 minutes, plus 2 hours freezing and chilling
Cook: 5 minutes
Makes: 5 cups syrup, serves 10
4 big slabs watermelon
2 cups water
2 cups sugar
Finely grated zest of 4 limes
2 cups freshly squeezed lime juice
10 fresh mint leaves
Sparkling water, optional
1. Freeze: Cut watermelon into neat 1-inch cubes. (No need to remove seeds — they're cute.) Set cubes on a rimmed baking sheet and freeze firm, at least 2 hours. Pile into a zip-close bag, and store in freezer.
2. Simmer: Measure water and sugar into a medium saucepan. Bring to a boil. Stir until sugar has dissolved and syrup turns from cloudy to clear, 3 to 4 minutes. Pull pan off heat. Stir in zest. Pour into a heat-proof jar and allow to cool to room temperature, uncovered. Transfer to refrigerator to chill, covered. When cold, stir in juice. Keep tightly covered in the refrigerator.
3. Serve: For each drink, pile frozen watermelon cubes into a tall glass. Add 1 mint leaf. Pour in equal parts water (or sparkling water) and lime syrup. Stir. Sip. Smile.