Cut off the tip and stem of the lime, halve the lime lengthways and then quarter each.
Put four or five lime eighths in a large tall glass, add the sugar and crush everything with a cocktail pestle. Then add the frozen raspberries and fill the glass with water. Stir vigorously with a long bar spoon and serve with a straw and decoration as desired.
If you like, you can crush a few leaves of mint together with the limes and sugar.