There's no telling how long the Chicago Teachers Union strike — the first strike of its kind in the city in a quarter-century — will last, but, as this photo clearly shows, the protests have already taken a turn for the dirty.

"Civility has disappeared in Chicago Teachers Union protests," says Daniel Strauss, who snapped the pic. Truly.

Make fun of the mayor's mother, declare him worse than Hitler, plant evidence that he's having an extramarital affair. But calling him a Nickelback fan? Unacceptable.

UPDATE: In an urgent email to the Chicago Tribune, a spokeswoman for Rahm Emanuel sought to assure constituents that the mayor is most definitely not a fan of Nickelback.

[H/T: HyperVocal, photo via @DanielStrauss4]