On January 15th Wikipedia turns 20. The online encyclopedia of 55m articles, edited by volunteers, is the world’s 13th-most-visited website and has become a standard online reference work—although it insists “Wikipedia is not a reliable source”. (The Economist’s fact-checkers frown on journalists relying on it.) Its intimidating rules for editors help to filter out dilettantes and ideologues, and its open nature means that errors in well-read articles are quickly fixed. And although the number of editors in rich-world languages has declined, the number writing in poor-country languages has been rising fast. Even so, the spread of accessible online knowledge is far from complete.