8 cups loosely-packed seeded watermelon chunks
1/4 cup packed fresh mint leaves
12 ounces sparkling water
Sugar
Ice cubes
Mint sprigs (for garnish)

In a blender, purée the watermelon in batches, adding the mint leaves to one of the batches. Pour the watermelon juice into a large pitcher. Stir in the sparkling water and taste. Add 1 tablespoon or more of sugar if needed. Chill for at least 1 hour. Pour over ice in tall glasses and garnish each glass with a mint sprig, if desired.

Variation
Substitute mangos or blood oranges for the watermelon.