Cube up a half of a small watermelon in order to get 3-4 cups of watermelon. Freeze watermelon, cubed, overnight on a flat surface.
In a food processor or blender, blend 3 cups of frozen watermelon cubes, the lime juice, and agave (add more if you prefer a sweeter drink).
Process until you have a slushy mixture, and then add the tequila and triple sec and blend for 5-10 seconds maximum. To thin the drink out further, add a bit of water - or tequila to strengthen the drink. Taste and adjust.
Pour the drink into your glass and top with extra frozen watermelon cubes for ice cubes.
Recipe by Uproot Kitchen at https://uprootkitchen.com/2014/04/30/watermelon-margaritas/