Muddle mint leaves, raspberries and sugar in a glass.
Add lime juice and rum and stir.
Fill glass with ice and top off with soda.
Garnish with a few berries and/or mint sprig if you like.
To serve to a crowd, increase the amounts of mint, raspberries, lime, sugar and rum, muddle them in a pitcher or cocktail shaker, then pour mixture into glasses before topping each drink with ice and soda.