It was supposed to be a birthday night out for the kids in San Francisco, but instead turned into a Critical Mass horror show -- complete with a pummeled car, a smashed rear window and little children screaming in terror. The spontaneous Critical Mass bike rides, in which thousands of free-spirited cyclists roam the city, have been a fixture on the last Friday night of the month since the early 1990s. But even bike-weary cops, who have seen their share of traffic disturbances and minor skirmishes, weren't prepared for what happened during the latest exercise of pedal power. Here's the...