In a cocktail shaker or jar with a lid, place the mint, lime and sugar (depending on how sweet you like your drinks use more or less). Using a muddler or large spoon, smash it all together until the sugar is dissolved. Pour in the rum.
Add a handful of ice cubes, place the lid on the shaker or jar and shake vigorously.
Have ready two low-ball glasses filled with ice and some fresh mint sprigs. Strain the juice and rum mixture into the glasses equally and top each with equal amounts of sparkling water.
If you don't have key limes, you can always substitute the juice of regular limes.