Fill a cocktail shaker halfway with ice cubes.
Add the gin, triple sec, pineapple juice, and grenadine.
Shake vigorously for 10–12 seconds until well chilled.
Strain into an old-fashioned glass filled with fresh ice.
Garnish with a pineapple slice on the rim and serve immediately.
Float the grenadine last over the back of a bar spoon rather than shaking it in - this preserves the layered sunset gradient effect in the glass before the drink is stirred by the guest.
My Bar Shelf reads your shelf and shows you every cocktail you can make from what you already own, including Grass Skirt.