Use a mortar and pestle to mash the mint leaves. Remove and discard the rind and seeds of the watermelon. Put the fruit into a food processor fitted with the steel blade and purée. Put the mashed mint into a large pitcher with 2 cups of puréed watermelon, the rum, simple syrup, and lime juice and stir to combine. Pour the mixture into a pitcher for serving.
Place ice cubes in 6 glasses and pour the mojito mixture into the glasses. Garnish with sprigs of fresh mint and spears of watermelon. Serve ice cold.
Copyright 2010, Barefoot Contessa How Easy is That? by Ina Garten, Clarkson Potter/Publishers, All Rights Reserved