1. Preheat oven to 425 degrees. Halve grapefruits crosswise with a grapefruit knife and cut out sections from between membranes. Transfer sections and any juices to a large bowl and stir in 2 tablespoons sugar. Gently peel away and discard inner membranes from grapefruit halves, leaving rinds intact, to form 4 hollow, dry cups. Divide grapefruit mixture among cups.