Bund, the German version of Friends of the Earth, created balloons that look like our planet printed with the text "The world can't take anymore C02." They attached the uninflated balloons to vehicle tailpipes around the city of Berlin. When the drivers got into their vehicles and started them, the balloons inflated until they burst, causing a loud bang, embarrassment for the drivers and attention to the problem of CO2 emissions into our environment.

Pigeon hopes that they targeted unnecessarily gas guzzling status symbols, like Hummers.

http://blogger.xs4all.nl/marcg/archive/2007/01/09/171574.aspx