To the jar of a blender add the chopped watermelon, sugar, juice from the lime and salt. Blend until thoroughly pureed, about 1 minute. Now comes the spice! I recommend adding 1 teeny sliver of cayenne pepper at a time, tasting and then adding if you'd like more. My batch wasn't super spicy--there was just a bit of kick at the finish. If you don't have a cayenne pepper, other alternatives are ground cayenne pepper, Sriracha or chile paste.