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Watermelon Agua Fresca
8 cups cubed seeded watermelon, divided
1 cup water, divided
1/3 cup granulated sugar, divided
1/4 cup fresh squeezed lime juice
1 liter seltzer water, pre-chilled
Puree half of the cubed watermelon, half the water and half the sugar in a blender. Pour through a coarse strainer into a large container. Repeat with the remaining watermelon, water and sugar. Stir in lime juice. Refrigerate until very cold - at least 4 hours.
Before serving, stir in seltzer and garnish with fresh lime slices if desired.