Set out 8 tall 12-ounce glasses. Into each glass strip off the leaves from a single sprig of mint—you’ll need about 10 leaves for each drink—and top with 1/2 cup watermelon cubes. Divide the Simple Syrup among the glasses (1 tablespoon - 1/2 ounce per glass). Use a muddler (or the handle of a wooden spoon or a long-handle ice tea spoon—though neither is anywhere near as effective) to crush the mint and watermelon, releasing their flavor into the syrup—the more muddling, the fuller the flavors. Fill each glass with ice. Measure in the rum (1/4 cup - 2 ounces per glass) and the lime juice (1 tablespoon - 1/2 ounce per glass). Use a long-handle ice tea spoon to mix everything together. Top off each glass with a little sparkling water or soda and you’re ready to serve.