This cute, two-minute video created by with the help of the SF Department of Emergency Management explains the history of San Francisco's outdoor warning system and profiles Cesar Santos, the dispatcher who tests the Siren every Tuesday at Noon. We wonder what Tray 3 is, as in "Don't Use Tray 3" in the YouTube freeze-frame.

And remember, if you hear the siren any other time besides Tuesdays at Noon, go indoors and immediately tune to a news source such as KCBS 740 AM, or another local media station. In a real emergency, the siren tone will cycle repeatedly for 5 minutes.