Process watermelon in a blender or food processor until smooth.
Stir together watermelon puree and remaining ingredients. Pour mixture into a 2-quart glass bowl. Cover and freeze 8 hours, stirring occasionally.
Remove from freezer 15 minutes before serving. Stir with a fork, and spoon into glasses. Serve immediately.
Note: For testing purposes only, we used 7 UP soft drink.
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