Simmer 1 cup sugar and 1 cup water until sugar is dissolved. Chill.
30 large, fresh mint leaves, coarsely torn by hand
3 or 4 thick slices of watermelon
12 oz light rum
1/2 cup simple syrup
6 Tbsp freshly squeezed lime juice (from about 3 limes)
mint sprigs and watermelon spears for garnish
Use a mortar and pestle to mash the mint leaves. Remove and discard the rind and seeds of the watermelon. Put the watermelon into a food processor and puree.
Put the mashed mint into a large pitcher with 2 cups of pureed 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. Garnish and serve.