If you've ever wondered what the strongest alcoholic drink you can have is, the answer lies in something called the "proof" of the alcohol. If you've ever looked at the label on a bottle of liquor, you've probably seen both the ABV (alcohol by volume) percentage and a "proof" number, often near each other on one of the lower corners of the label. But what do these numbers mean, and how did proof become a standard way to measure alcohol's strength? It's actually a pretty cool story.

The proof is more than just an extra number on the already busy labels of alcoholic beverage bottles, although it feels that way if you're not sure what it means. Instead, it's a nod to the history of alcohol testing and it gives us a way to understand a drink's strength before we try it and make the pursed-lip face akin to someone who bit into a lemon. Originally, "proof" was a method of testing spirits' alcohol content. In the 18th century, the British Navy would pour gunpowder on alcohol and ignite it; if the mixture burned, it was "proof" of high alcohol content. Today, proof has evolved to a precise, standardized way to measure alcohol strength that doesn't include any gunpowder at all. In the U.S. it's exactly double the ABV, so if your favorite rum reads 40% ABV, it's 80 proof, which makes it simple to read and understand.