Add the cubed watermelon to the base of your blender along with lemon or lime juice if using. Blend until smooth (this should only take a minute).
Strain the juice to remove the pulp. To do this, place a nut milk bag or several layers of cheesecloth over a large bowl and pour the juice through, squeezing out as much of the liquid as you can.
Store in a tightly sealed jar in the fridge until ready to consume. It'll naturally separate the longer it sits, so just shake or stir before serving. Best served cold or over ice.
VARIATIONS:Fizzy drink - add sparkling water (or kombucha) and ice.Cucumber - add sliced cucumbers to the blender for an even more refreshing twist.Strawberry - add in fresh sliced strawberries or blend them up with the watermelon before straining.Basil watermelon drink - add basil simple syrup, sparkling water (or alcohol if desired), lemon juice, and ice