There’s a simple structure behind all sours, a family of citrus-based cocktails.
Combine rum, lime juice, and simple syrup in a cocktail shaker. Fill shaker with ice, cover, and shake vigorously until outside of shaker is very cold, about 20 seconds.
Strain cocktail through a Hawthorne strainer or a slotted spoon into a coupe glass. Garnish with lime twist.