The old training shops 'black museum's' contents, photo's mainly, but the odd irrecoverable finger stuck inside a nut in a jar like in the one I trained at, did a pretty good job of showing the consequences of stupidity, banned now. But there are still plenty of examples on youtube, the simplest sign I ever saw was "this machine can remove hardened steel at astonishing speed, do you really think your puny soft flesh and bone stands any chance?"