`   Some soda fountains of the fifties kept a bottle of ``phosphate`` near the spot where soft drinks were concocted, and a customer could order, for example, a cherry coke with phosphate.  One drop would add some tartness; two drops resulted in a major pucker.  Cherry cokes were a prime candidate for this treatment as the cherry flavoring lent an abundance of sweetness.  The phosphate, whatever it was, was a clear thin liquid.`