It's not exactly a spontaneous flash mob. There's some lead time. But it's public mayhem -- with all natural materials. It happens here, in London, in New York, in Tel Aviv, to name a few spots.

Our version, the Great San Francisco Pillow Fight happens this Thursday, Valentine's Day, at Justin Herman Plaza. When the clock strikes 6p, if you have a pillow you can pummel anyone who has a king-size sham to fight back.

Organizers ask that you bring pillows stuffed with natural materials only. Synthetics can make their way into the bay and harm wildlife. And consider bringing a few trash bags to help clean up.

The San Francisco Food Bank and Coalition on Homelessness will be collecting donations this year -- pillows, blankets, clothing and non-perishable food items.

See photos from the 2008 San Francisco Pillow Fight: Valentine's Day Pillow Fight Images

See details, video and pics at the Pillow Fight website.

^{Image Courtesy of Beatrice Murch - Flickr Creative Commons}