Next to lemons, watermelons are my favorite fruit. Their distinctively light, sweet flavor seems to expand in my mouth with a refreshing zeal. They’re probably the only thing I love about summer. I make these when Tom has some Southwestern chicken on the grill. The trick is to use seedless watermelons so you can cut them into chunks and toss them directly into the processor. The lime juice enhances the melon; from the moment I add it into the blender, I begin anticipating the combination of tart and sweet. Don’t be afraid to improvise. The fruity twist on this standard cocktail opens up a world of possibilities. Think of it as a vacation in a glass.