Start here: If the liquid could be explosive, why are you dumping it in a crowd? And then go here: Could the liquid threat be hydrogen cyanide?
Not ONE terrorist was caught by making him dump liquids in a bin with thousands of other people. These terrorists were caught by police work and effort. Not only is the mass dumping of liquid utterly ineffective, it's possibly REALLY dangerous if any terrorist is actually carrying explosives/poison/whatever.
The "leadership" of this country is just trying to keep us all terrified with the hopes that we'll re-elect them because of their tough, but utterly useless, stance on the "war on terror". How many rights do we have to give up to the neocons before the sheeple fight back?