Add the vodka, triple sec, and grapefruit juice to a cocktail shaker filled halfway with ice. Shake for 10-15 seconds, or until very cold. Strain into a tall glass filled with crushed ice.
Top with lemon-lime soda or sparkling water.
Garnish with a slice of fresh grapefruit.
Notes: Pamplemousse La Croix is my favorite sparkling water to use in these cocktails.