Mural people are nuts. Wall paintings never last and can't be purchased, and the people who go specifically to see them are often the hoi polloi who know nothing about art. How can muralists expect to survive in the art world? The answer: They don't. The folks who make their work on outdoor surfaces tend to be little-D democrats who love the idea of art for art's sake, who like bringing joy to all kinds of passersby, from tiny stroller inhabitants to seniors whose gait may give them more time than anyone to appreciate what's right in front of them. The weekly Mission Trail Mural Walks give you, whatever your age and whatever the state of your art education, a chance to see the pieces that famously decorate the Mission District with a guide who can answer your questions, even if yours is, "Don't these people know they're never going to make any money doing this?"