Why is this joke funny? The circumference of a circle equals $2\pi r^2$. [closed]

What does Randall from XKCD.com mean with this joke?

http://xkcd.com/1184/

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closed as off topic by Alex Becker♦, Matt Pressland, copper.hat, Jasper Loy, Ross MillikanMar 12 '13 at 16:30

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An example of bad labelling, I guess. –  J.H. Mar 12 '13 at 16:23
The exponent is supposed to be a citation. But I'm not sure if that makes it funny. –  Alex Becker Mar 12 '13 at 16:23
explainxkcd.com –  Aaron Blenkush Mar 12 '13 at 16:56

$r^2$ looks like $r$-squared. But in fact the exponent $2$ is a footnote mark.
Well...it's certainly not the best cartoon of XKCD. I guess the exponent $2$ was chosen, because it makes the formula very close to the formula for the area of a square (up to the factor $2$). –  azimut Mar 12 '13 at 16:29