Good luck getting across Downtown on the last Friday of the month, when what cab drivers ruefully call "Critical Mess" — equal parts political protest, bike ride, and rolling party — joyfully locks up San Francisco's streets. You have to ride in one to understand the euphoria of the event, which usually snakes around the city, stopping traffic for a few hours before the riders pause to grab a beer at the unofficial Zeitgeist after-party. And while some people gripe about it, Critical Mass has only gotten bigger in its 13-year history: It has now spread to every continent except Antarctica. Surrounded by thousands of Bay Area cyclists, you'll find that it's one of the most entertaining ways to see San Francisco by bike, not to mention get some environmentally conscious exercise. If you happen to be one of the unlucky ones who has to sit in his car while the fun rides by, it's best to just enjoy the view from the sidelines; it'll be a while before you go anywhere.