Chocolate's history began in ancient Mesoamerica. The Olmecs, Mayans, and Aztecs processed cacao seeds into a bitter, frothy drink often mixed with chili peppers and vanilla. Cacao was so valuable that the Aztecs used the beans as currency. Spanish explorers brought chocolate to Europe in the 16th century, where it was sweetened with sugar and cinnamon. The transition from a drink to solid bars happened in the 19th century. In 1828, Casparus van Houten invented the cocoa press, which separated cocoa butter from the solids. In 1847, Joseph Fry created the first chocolate bar by mixing cocoa powder, sugar, and cocoa butter. Later, Daniel Peter and Henri Nestlé added condensed milk to create milk chocolate. The unique 'melt-in-the-mouth' quality is due to cocoa butter having a melting point just below human body temperature.