Place 10-12 fresh mint leaves, half a lime cut into four wedges and 2 tsp caster sugar in a tall glass.
Gently muddle the mint, lime and sugar with a muddler or the end of a wooden spoon, pressing about 5-7 times to release the mint oils and lime juice without tearing the leaves.
Add 60 ml white rum and stir briefly to help dissolve the sugar.
Fill the glass almost to the top with ice cubes.
Top up with 150 ml soda water and stir gently to combine without losing the fizz.
Garnish with an extra sprig of fresh mint and a slice of lime, then serve immediately.