Dice 1/2 a watermelon into large chunks and place into a blender.
Measure out the sugar and pour into blender.
Juice 1 large lime and pour juice into blender.
Blend until smooth, and pour into a 9 x 13 inch baking dish.
Place in freezer for ~2 hours. As liquid begins to freeze, rake with a fork. Return to freezer and stir every 2 hours, breaking up clumps and creating ice crystals. Rake once more, just before serving.
Serve in glassware, ice cream cups, or a scooped out watermelon half.
Can be stored up to 4 days in a covered container in freezer.