A strange earthy, minty soft drink available in three colors without any difference in flavor is the preferred drink of millions of Pennsylvanians. What is birch beer, and what makes it so good?Perhaps the defining quality of the cuisine of southeastern Pennsylvania, where I grew up, is a fierce opinion about small differences. An Italian hoagie must not, under any circumstances, contain mayonnaise; it must have olive oil. An Italian hoagie with mayo is incorrect. Also incorrect: a soft pretzel in a traditional pretzel shape. Philadelphia soft pretzels have a unique thin, rectangular, symmetrical shape with the knot right in the middle. A regular round soft pretzel is wrong — or worse, from New York. And so it is with the soda preferred by Pennsylvanians, including a strange reddish herbal soda I grew up drinking and didn’t realize was odd until I left.