The death of a Wal-Mart security guard in New York on Black Friday has Americans lamenting the human condition. The shoppers at the Valley Stream store have been called "rabid"...Wal-Mart, meanwhile, has claimed it prepared for the crush by bringing on outside security...But for people who study crowd crushes, there was nothing surprising about what happened at Wal-Mart..."We know exactly how crowds work," says G. Keith Still, a crowd management expert...There is, he says, no excuse for these kinds of accidents. "It's stupidity. It's ignorance. But the consequence is human life."...The investigation into the death of Jdimytai Damour, 34, is ongoing, and...[a] lawsuits have begun...But here is what we do know about crowds, generally speaking...