Wash the lime hot, grate dry and cut into eight pieces. Wash mint, dab dry, pluck leaves from the stalks except for 1 tip of the stem.
Lightly mash lime and sugar in a glass with a cocktail masher. Add rum and mint leaves. Press the leaves lightly or rub the edge of the glass (do not crush!)
Add crushed ice to about 2/3 of the height of the glass, fill up with soda water, stir again and decorate with remaining mint.