Fill a cocktail shaker with ice. Add cointreau, tequila, lime juice, and simple syrup. Cover and shake until mixed and chilled, about 30 seconds. (In general, the drink is ready by the time the shaker mists up.) Strain margarita into the glass.

SIMPLE SYRUP: Put the sugar and water in a small saucepan. Cook over low heat, stirring, until the sugar dissolves. (Alternatively, heat in the microwave.) Store in a sealed container in the refrigerator for up to 1 month.