Cover lemons with a little water, soften. Add sugar and sage, stir, make lightly for 1/2 hour with the lid closed.
Filter through a gauze cloth into a large container. Add water and beer, stir.
When cooled, pour into clean swing-top bottles, seal tightly.
Stand in the sun or at room temperature for two or three days until the sparkling beverage is formed. Keep in a cool place, drink within a week or two.
Tips Fill bottle only to five cm below rim The longer the drink is left to stand, the more it bubbles.
Do not shake bottle, open carefully.
Caution: do not leave the bottles in the sun for too long, they may break under the pressure of fermentation.