Who knew?

I confess to not having a clear idea of what a "controlled explosion" is; i.e. when the constabulary perform one after finding a suspect package in a Tube station. I'd always imagined that it meant that they took the bag out and blew it up. If it made a big bang it was a bomb and if it made a small bang it was someone's insurance claim.

It turns out that it is rather more scientific than that.