In a 1 1/2- to 2-quart glass measure or bowl, combine basil leaves (see notes) and sugar. With a wooden spoon, crush leaves with sugar until thoroughly bruised. Add water and lemon juice. Stir until sugar is dissolved, 1 to 2 minutes. Taste and add more sugar if desired. Pour through a fine strainer into ice-filled glasses. Garnish with sprigs of fresh basil.