Ancient Romans used pee as mouthwash and toothpaste to get a bright white smile.

The secret behind this gross-sounding habit is a chemical called ammonia. When urine is left out, tiny bacteria break down a substance called urea and turn it into ammonia. This makes the liquid very alkaline, which means it is great at dissolving fats and proteins that cause stains. We know about this because of a Roman poet named Catullus who lived over 2,000 years ago. In one of his poems from around 54 BC, he teased a man for having teeth that were too white, claiming it was because he used so much pee. This wasn't just a weird DIY project. It was a huge business in the Roman Empire. Urine was collected from public toilets and used for many things. In 70 AD, Emperor Vespasian even put a tax on it to make money for the government. Professional cleaners called 'fullones' used it to bleach clothes and get tough stains out of wool. Romans were very picky about their supply. They believed that urine imported from the Iberian Peninsula, which is modern-day Portugal, was the strongest and best for whitening. While we use much cleaner chemicals today, the ammonia found in old pee is actually a distant relative of the ingredients used in modern glass cleaners and some teeth whiteners.