Cut the watermelon flesh from the skin, slice thinly and remove seeds as much as possible. Freeze the fruit flesh in a flat container for at least 1 hour. Wash the mint, shake dry and pluck leaves from the stalks except for a little bit for decoration and cut into strips. Remove frozen fruit flesh from the container and chop roughly with a knife. Sprinkle with mint and sugar and sprinkle with rum and lime juice. Mix and distribute on frozen glasses. Decorate with mint
1 hour waiting time