Cut a round slice of lime, 1 cm thick and slice it into 4 equal parts. Put the four lime wedges into a glass, then add the sugar and muddle (squish everything together) to release the lime juice and aroma. Do it gently because hard squishing may release the bitterness from the white part of the lime.
Put the mint leaves on one hand and clap in order to lightly bruise the leaves to releases the aroma. Rub the mint leaves around the rim of the glass and drop them inches Use a muddler, bar spoon (or even a rolling pin) to gently push the mint down into the lime juice.
Half fill the glass with crushed ice and pour in the rum. Stir the mix together until the sugar dissolves.
Top up with crushed ice, a splash of the sparkling water and garnish it with a sprig of mint.