Three Degrees of Wikipedia
If you go to Wikipedia, and try to navigate from one article to another you will probably learn plenty about both subjects. More importantly, you are very likely to remember the learning because our memory works through connections. Created by Ivan Zhao, Three Degrees of Wikipedia makes this into a social game. You can not only acquiring new knowledge through solving such 3-degrees connection puzzles, but also enlightening other players by create your own puzzles.
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