SPARKLING GINGER-LEMON SODA

Steps:

• Combine ginger and 2 quarts cold water in a large pot. Cut lemons in half and squeeze into pot, straining out pits. Drop lemon halves into pot and bring to a boil. Turn off heat, cover and let steep 30 minutes. Stir in honey, then taste for sweetness, adding more to taste. Strain mixture into a container and discard solids. Let cool to room temperature, then chill until very cold.
• Working in small batches, pour cold mixture into the bottle of a carbonator, filling it not more than 1/3 full (this will give the mixture room to foam). Charge with CO2 until very fizzy, waiting between charges for foam to subside completely. (This can take up to 2 minutes, so be patient.)
• Very slowly, as if opening a bottle of soda, release the bottle from the carbonator. Serve immediately, or cover tightly and chill. (Extra soda mixture can be refrigerated for up to a week, and can be served carbonated or flat.) To serve, pour over ice and top with a mint sprig. If desired, add a slug of rum to each glass.