Gently muddle 2 lemon slices, 2 Tbsp. simple syrup, and 1/2 tsp. root beer extract in a 10-oz. glass. Fill glass with crushed ice, and top with seltzer water or club soda. Gently stir. Garnish with a fresh lemon slice.
Extra Point: Make your own simple syrup by cooking equal parts sugar and water over medium-high heat until sugar dissolves. Chill until ready to use.