Add blackberries and torn mint leaves to a glass with lemon juice, lime juice, and simple syrup.
Muddle together to break up the berries and mint.
Add the rum and fill the glass with ice.
Pour sparkling water to fill the glass.
Stir to combine and garnish with more berries and mint.
Serve immediately.
Notes
Add as much or as little simple syrup as you like, to make this drink more or less sweet.It's easy to make your own simple syrup - just boil together equal parts sugar and water until the sugar has dissolved.This mojito is just as delicious with raspberries, blueberries, and strawberries!