If you drive through the Northwoods or anywhere in rural America, you'll notice most old barns are red. While many assume it was just a stylistic choice, the real reason is practical, and a bit scientific. In the 18th and 19th centuries, farmers needed a cheap and effective way to protect their wooden barns from rot. They mixed their own paint, often using skimmed milk, lime, and rust (ferrous oxide). The rust acted as a preservative, preventing mold and decay. As a bonus, it gave the barn its distinctive deep red color.