1. Place a 20 x 30 cm (8 x 12") shallow metal baking pan in the freezer.
2. Combine the water, sugar and grapefruit zest in a medium saucepan. Add the cinnamon sticks. Bring to the boil, stirring until sugar dissolves.
3. Reduce heat and simmer, without stirring, for 3 minutes. Remove from heat. Stir in grapefruit juice. Cool to room temperature. Remove cinnamon sticks.
4. Pour grapefruit mixture into chilled pan and return to freezer. Freeze for 3 hours, removing from freezer every 30 minutes to scrape with a fork to break up ice crystals. When set , the granita should be the texture of soft crushed ice. Scoop into serving cups and top with grapefruit segments.

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Photo and suggestion: Coles, your supermarket in Australia