Critical Mass
Once a month – typically on last Fridays' afternoons – diverse groups of people on bikes hit urban squares and streets all over the world to share a ride through their cities, get in touch, and keep up with what's happening. I had the luck to attend Critical Mass rides in Santiago, Magdeburg, and Calgary and always encountered friendly people.
The stencil states in Spanish slang: on a bike you're prettier (photo by ⌠david⌡ on Flickr).