Bloody Coasters

This is a great example of how simple advertising can be very effective.  These coasters were put out by the Mumbai Traffic Police to help prevent drinking and driving (drunken driving as they call it).  The coasters have a layer of invisible red ink that is activated by liquid.  When you pick up your glass the ring of moisture on the coaster turns red, creating the illusion of blood over the image.  (Found through Houtlust)