How to crush steel cans without touching them...

If you have a sealed can of air, and you rapidly cool it, then the pressure inside will drop. 

If it drops far enough then the pressure from the hundred-or-so kilometres of Earth's atmosphere above (which is about the same as the weight of a minivan) will do this to it.....

My thanks to the 'Doctor, Doctor' team. No steel cans were harmed in the making of this. 

Apart from the one that got squashed, obviously.