lemon and lime mintade

In a medium saucepan, bring sugar and water to a simmer, stirring until sugar dissolves. Remove from heat and add zest (just the yellow and green part of the skin, not the white underneath) of both lemons and limes and all but a few leaves of the mint (save them for garnish). Cover with a lid and let chill completely, about 1 hour in the fridge. Meanwhile, juice your lemons and limes. You want 1 cup total, half lemon and half lime juice. Once mint-zest syrup has chilled, strain out the solids and add the lemon and lime juice to the syrup. You can chill this juice-syrup mixture until needed, or up to one week in the fridge.To serve: Fill a medium-sized glass with ice. Fill halfway with juice-syrup mixture, and the rest of the way with seltzer. Garnish with reserved mint leaves. Drink immediately.