Prepare the Glass: Rim a martini glass with sugar by dipping the rim in lemon juice and then in sugar. Set aside.

Shake: In a cocktail shaker, add vodka, grapefruit juice, lemon juice, simple syrup, and ice.

Mix: Shake well until the mixture is chilled.

Serve: Strain the mixture into the prepared martini glass.

Garnish: Add a grapefruit twist or a lemon slice for garnish.