The word 'soap' comes from the Roman legend of Mount Sapo.

The legend of Mount Sapo was first popularized in the 18th century, though it likely reflects much older oral traditions. According to the myth, the mountain was a site for animal sacrifices where fat dripped into the wood fires. When rain fell, the resulting mixture of rendered tallow and potash-rich ash flowed down the slopes and into the clay of the Tiber River. Roman women washing clothes downstream noticed that this specific clay made their laundry significantly cleaner with less effort. Chemically, this describes the process of saponification, where a triglyceride (fat) reacts with a strong base (alkali) to produce glycerol and a fatty acid salt, which we call soap. While the Mount Sapo story is widely considered a folk etymology, the chemistry it describes is scientifically accurate. The first written mention of soap-like substances dates back to ancient Babylon around 2800 BC. However, the Roman scholar Pliny the Elder wrote about soap in his work 'Naturalis Historia' in 77 AD. He credited the Gauls and Germans with its invention, noting they used it to give their hair a reddish tint. Despite the mythic nature of Mount Sapo, the Latin word 'sapo' is indeed the root for the English word 'soap' and the Italian 'sapone'. Modern soap production still relies on the same fundamental interaction between fats and alkalis. Today, manufacturers use sodium hydroxide or potassium hydroxide instead of wood ash to ensure consistency and safety.