Add simple syrup into a pitcher and add grapefruit juice and lemon juice.
Serve cold over ice; top with sparkling water. Garnish with lemon.
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Notes
Add up to another ounce or more of simple syrup to taste.To make simple syrup: cook 1 cup sugar and 1 cup water in a small saucepan over medium heat. Add mint and cook until just simmering, cool then strain. Store in fridge.