Combine 1 cup each lime juice, water and sugar, and 1/4 cup torn mint leaves in a medium skillet; cook over medium-high heat until syrupy, about 20 minutes. Stir in 1/4 cup more torn mint leaves; let cool, then strain. Add seltzer.
For each drink, combine 2 to 4 tablespoons flavored syrup with 1 cup cold seltzer in an ice-filled glass.
Photograph by Charles Masters