Take any article on Wikipedia. Who wrote it? Where did it come from? Now take a closer look at those unconvincing, badly written sentences in the middle. Why did someone add them? How long will it be before someone else deletes them? And how many people will have read them before they are removed? Five years ago such questions didn't matter; Wikipedia was one source among many, and no one took it very seriously. Two years ago they hardly mattered, because the newspapers said Wikipedia couldn't be trusted, and there was always a more 'reliable' source to check later. But suddenly, these questions really do matter. With all its nonsense, its illiteracy and its unreliability, Wikipedia is currently the eighth most visited site on the web. Whatever they say, most people rely on it most of the time. Those other sources won't be around much longer, and Wikipedia will be the best there is. But is it good enough to rule the world of knowledge? And how big will it be ten years from now?