Peel the lemons and chop the peel in to tiny pieces. Squeeze the lemons to make the juice and put to the side.
Make a syrup by boiling the chopped lemon, with the water (just cold from the tap, not the fizzy water) and the sugar for 5 minutes.
Pour the syrup in to a container, mix with the lemon juice and the fizzy water.
Pop in the freezer, but stir it every hour for about 6 hours, so that it freezes like sorbet!