Cut watermelon into chunks to yield around 8-10 cups. This makes enough for about 8 mimosas.
Add watermelon chunks and orange juice into a blender. Blend together.
With a fine-mesh strainer, strain out the pulp.
To make a mimosa, add 3 ounces watermelon juice to a champagne flute and top with champagne. (If using triple sec, use 2 ounces watermelon juice and 1 ounce triple sec, then top with champagne.
Notes
No orange juice? Use lime juice, lemon juice or even water. Sparkling wine tips: Prosecco, cava, asti and spumante will all work for mimosas if you don't want to splurge on champagne. Use a drier sparkling wine to pair with sweet melon. Look for "brut" on the label.