Fill 2 highball glasses with ice cubes.
In a cocktail shaker or large, lidded jar, shake the vodka, lime juice and pineapple juice together with a handful of ice cubes.
Strain into the highball glasses.
Slowly add the grenadine and stand for a minute or so while the grenadine settles into a pink layer at the base of each glass.
Add a slice of pineapple to each drink before serving.
