On May 22, 2010, Bitcoin was used for the first time as a unit of account in a real-world transaction when Laszlo Hanyecz exchanged 10,000 BTC for two pizzas. This event, now celebrated annually as Bitcoin Pizza Day, marked the first documented instance of Bitcoin fulfilling the 'unit of account' function, one of the three primary functions of money. Prior to this, Bitcoin had an exchange rate but lacked a market price. The transaction highlighted Bitcoin's evolving role in price discovery mechanisms, transitioning from cost-of-production anchoring to peer-to-peer matching and eventually to centralized matching with continuous quoting. This evolution mirrors historical asset classes, such as the Dutch East India Company shares and 19th-century grain futures, showcasing a structural similarity in the development of price discovery mechanisms across different eras and asset types.