Squeeze lemons to get about 1 cup of juice. Combine lemon juice and sugar in a saucepan over medium heat. Stir until sugar dissolves, about 5 minutes. Remove from heat and let cool.

In a 12-ounce glass, pour 4-5 tablespoons of the cooled syrup. Fill with ice cubes, leaving space at the top.

Pour 1 cup of sparkling water over the ice and syrup. Do this slowly to preserve carbonation.

Garnish with lemon slices and mint sprigs for an appealing presentation.

For a pitcher, combine the entire batch of syrup with 4-5 cups of sparkling water in a larger pitcher. Add lemon slices and mint for sharing.