Mash the insides of the watermelon with a potato masher until liquified (you're looking for 2 cups of liquid; small pieces of watermelon in the liquid are fine). Note: You can scoop out the seeds, but they look beautiful in the frozen pops. Add pure cane sugar to watermelon and stir until the sugar completely dissolves. Stir in mint leaves. Pour mixture into a popsicle mold. Wait for it to partially freeze, then insert sticks and freeze completely.