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.