To a cocktail shaker filled halfway with ice, add tequila, fresh watermelon juice or purée, the juice of one lime, agave nectar, and fresh mint.
Add the lid and shake until the surface of the cup is icy.
Fill a rocks glass halfway with ice.
If you like, cut a slit into a small slice of watermelon to add as a garnish.
Strain the watermelon mixture into the glass.
