Diamond Sue walks up to the bar (at Lewis Brothers Home Made Ice Cream Shop on Commercial Street in Provincetown,) and asks for a drink. The fellow behind the bar says, "We sell ice cream here." (He's thinking, "What is this, a joke?") Diamond Sue insists on a drink, so the ice cream man finally asks, "What do you want to drink?" And Diamond Sue, to his surprise, asks for an egg cream. "We don't have that," he says, "but I can make it up for you if you tell me how, and I'll charge you the same as a soda." So Diamond Sue shares the recipe for an Egg Cream with the "bartender", while Marash Girl listens: "The drink (so delicious) is called an egg cream (in name only - no egg, no cream!). It's made with some chocolate syrup, milk and seltzer - soooo good!" And it cost Marash Girl (who was treating) the same as a soda (or as we say in New England, a tonic)!
N.B. Diamond Sue says that if you're in the mood, you can even add ice cream!