Cut a medium-sized watermelon into small pieces, removing the seeds.
Blend the watermelon pieces in a blender until smooth.
Strain the watermelon juice into a large jug, discarding the pulp.
Add 60 ml of vodka to the jug and stir well.
Add 1 tablespoon of lime juice and 2 teaspoons of sugar, stirring until the sugar is dissolved.
Pour the mixture into glasses filled with ice cubes.
Garnish with a slice of watermelon or a sprig of mint, if desired.