Pour 50ml rum into each of two highball glasses half filled with ice.
2 tbsp boiling water to dissolve the sugar 1 lime, sliced in half, is rubbed around the rims of the glasses with the cut side. Squeeze half of the lemon into the sugar mixture, then divide among the glasses.
Fill the glasses with 150ml chilled tonic water each after slicing the other lime into thin rounds and adding it to the glasses. To serve, give it a good stir.