<p>Saturday afternoon in Dolores Park feels bucolic. Dog owners walk their pets past groups of friends clustered around benches, chatting. Strolling couples steer around sun-lovers sprawled on the grass.</p>
<p>Chaos erupts at exactly 2:07 p.m. Sun-bathers suddenly leap to their feet and friends break formation. Strangers begin grabbing one another's hands and running toward a central point in the park. Pedestrians on Dolores Street and patrons of a sidewalk cafe charge up the hill to join them.</p>