1 10-lb. watermelon, seeded and chopped
1 1/2 cups fresh grapefruit juice
Working in batches, purée watermelon in a blender until smooth. Pour into a large bowl and let stand for 10 minutes. Skim foam from surface and discard.
Set a fine-mesh sieve over a large pitcher; line sieve with cheesecloth. Strain purée into pitcher. Stir in grapefruit juice.
Divide among ice-pop molds, insert sticks, and freeze until solid, at least 6 hours.
Makes 16 popsicles.