You all know the game "Galgenmännchen" (Hangman or find letters). This is what my idea is. With a piece of chalk greenpeace paint lines on the streets. That can be different Words or sentences with a special message. The people can scan that with their smart phone and they automatically linked on the homepage of greenpeace (SpecialMicrosite). They have to answer some questions to find the right letters. And they can read important facts. They can share it on facebook and sign an online petition.
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All pictures are only to show my idea. Background picture is from here: http://highresolution.photography/meeting/ and the iphone is a free mockup. http://all-free-download.com/free-psd/download/freepsdboardletters165230.html