Preparation

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.