Peal zest off lemons and limes (being careful not to get too much pith). Juice the remaining fruit, reserving pulp. Discard spent shells.

Put everything in a sauce pan. Bring to a boil and then simmer gently for 10 minutes, stirring occasionally.

Remove from heat and let cool.

Strain into container for storage.

Mix 1 part syrup to 4 parts carbonated water.