"Every time Bruce Schneier writes a fully general halt-checker, God kills a passenger pigeon. This is why passenger pigeons are extinct."
Bruce Schneier facts. Like Chuck Norris facts, only better.
This, of course, is Schneier's actual site. Less humorous, perhaps, but a great read.
On a somewhat related note, an interesting discussion of the feasibility of the liquid explosives plot, by a chemistry student. Read and think.