Put the rhubarb, sugar, lemon juice and 350 ml (12 fl oz) of water in a non-corrosive saucepan over a medium heat. Bring to the boil, then reduce the heat and simmer, uncovered, for 5 minutes, or until the rhubarb begins to collapse. Remove from the heat and leave to cool for about an hour.
Strain the rhubarb, reserving the liquid and discarding the pulp. Chill the rhubarb liquid for about 45 minutes.
Combine equal amounts of rhubarb liquid and sparkling mineral water in each glass, and serve.
Variation: for an alcoholic cocktail, add a dash or two of Pernod, which will impart a subtle aniseed taste.