Take a small, seedless, ripe watermelon and remove the rind. Cut the watermelon into small chunks and throw them into a blender. Blend until smooth and strain through a fine mesh strainer. Refrigerate the watermelon juice until ready to use.

Run a lime wedge over the rims of two rocks glasses and coat with tajin or salt. Fill glasses with ice.

In a cocktail shaker, combine the watermelon juice, tequila, grand marnier, lime, and simple with a little bit of ice. Pop the lid on and shake well for about 20 seconds. Strain into the prepared glasses and serve with a lime wedge.