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Rickrolling is an Internet meme involving the music video for the 1987 Rick Astley song "Never Gonna Give You Up". The meme is a bait and switch: a person provides a Web link they claim is relevant to the topic at hand, but the link actually takes the user to the Astley video. The URL can be masked or obfuscated in some manner so that the user cannot determine the true source of the link without clicking (and thus satisfying their curiosity). When a person clicks on the link given and is led to the web page he/she is said to have been "Rickrolled". By extension, it can also mean playing the song loudly in public in order to be disruptive.
Most believe the practice began as a variant of an earlier prank originating from the imageboard 4chan called duckrolling, in which a link to somewhere (such as a specific picture or news item) would instead lead to a thread or site containing an edited picture of a duck with wheels. The user at that point, is said to have been "Duckrolled". The first instance of Rickroll occurred on the site's video game board, where a link to the Rick Astley video was claimed to be a mirror of the first trailer for Grand Theft Auto IV (which was unavailable due to heavy traffic). The joke was confined to 4chan for a very brief period.
By May 2008, the practice had spread beyond 4chan and become an Internet phenomenon, eventually amassing some coverage in the mainstream media. An April 2008 poll by SurveyUSA estimated that at least 18 million American adults have been Rickrolled.
So next time you see some bullshit about Obama on the internet, you know know where to link to!