2 cups frozen watermelon cubes
1/2 frozen banana
1/2 cup water (plus more, if needed)
Juice of 1/2 lemon or 1 lime
1-2 tablespoons maple syrup or other sweetener (like honey)
Put all the ingredients in a blender or food processor and process until the mixture is smooth and frosty. You may need to add a bit more water at a time to loosen up the mixture. I found it liked to get stuck in my not-so-powerful blender. Try not to over water it or it will lose some flavor. Serve immediately (in frosted glasses, if desired!).