Cut a small seedless watermelon into 3-by-1/2-inch sticks. Mix 1 teaspoon chili powder with 1/2 teaspoon each ground coriander and cumin and 1/4 teaspoon kosher salt. Sprinkle all over the watermelon. Stir 1/2 cup sour cream with the zest and juice of 1 lime and a pinch each of ground cumin and salt. Serve with the watermelon fries.