I'm going to risk getting this one deleted, but this is all public domain.
Magnetometers (the arches you walk through) detect mainly ferrous steel. They can detect other metals but at much lower sensitivity. The spring steel wires in bras are detected, stainless steel watches are not. Titanium is very hard to detect.
The arches are set to randomly tigger about every hundred people, whether you have metal on you or not.
As Bruce Schneider points out - while the worst penalty you get for being caught with a knife, is having it confiscated - there is no reason to not keep trying until you suceed. It's all theatre.
Simon.