1 quart water
1 cup sugar
1 cup fresh lemon juice
sparkling water (not tonic water, like Perrier)
Mix first three together. Fill a tall glass to 3/4 with lemon mixture then fill with sparkling water.
An interesting variation is to puree some fruit (raspberries, strawberries, etc.) with a little superfine or powdered sugar and put that in the glass before adding the lemonade and water.