There are so many things to celebrate about Wikipedia, on the occasion of its 10th birthday, starting with its incredible breadth -- where else would you go to find out who Big Bill Neidjie was, or where Lower Carniola is? -- but the thing I love most is what it tells us about the way it changes our view of the world, around issues like collaboration, generosity, or authority.
A common complaint about Wikipedia during its first decade is that it is "not authoritative," as if authority was a thing which Encyclopedia Britannica had and Wikipedia doesn't. This view, though, hides the awful truth -- authority is a social characteristic, not a brute fact. Authoritativeness adheres to persons or institutions who, we jointly agree, have enough of a process for getting things right that we trust them. This bit of epistemological appraisal seems awfully abstract, but it can show up in some pretty concrete cases.
DARPA, the Pentagon's famous R&D lab, launched something in late 2009 called "The Red Balloon Challenge." They put up ten red weather balloons around the country, and said to contestants "If you can tell us the latitude and longitude of these balloons, within a mile of their actual positions, we'll give you $40,000." However, because the Earth is curved, DARPA also had to explain the Haversine forumla, which converts latitude and longitude to distance.