You're probably wondering what Castile soap is, exactly. Yeah, same. And how did it become the staple in this soap-making family's stable of goods? Well, it's a bit like the history of Champagne . . . sort of. According to the Pharmaceutical Journal's "A Short History of Soap" and the Dr. Bronner's archives, in earlier centuries, all-vegetable-oil-based soaps were made out of local olive oil in the Castile region of Spain. By the turn of this century, "Castile" became what any vegetable oil-based soap was called. The Heilbronners invented the first liquid version of Castile soap in the late 1800s to supply washrooms across Germany. And then, in 1948, Dr. Bronner started making the liquid peppermint version that has become iconic.