I had the same fear RH, it's really simple and safe with a modern pressure cooker. Multiple relief valves. Just follow the simple directions for your altitude, cooking time and pressure. If you pull a jar off the shelf and you can pop the lid before opening chuck it. The thing I found interesting is it'll continue to boil in the jar after pulling them out of the pot for half an hour. Let the pressure drop on it's own before opening the cooker lid, I guess would be an additional tip, but it's really all in the booklet you get when you buy a cooker. Check out that link I posted earlier, pretty informative.
Old science trick to get kids thinking was to boil water in a flask by pouring hot water in the flask and rubbing it with snow (or ice) and watching as the snow makes the water boil...
The boiling point of water is pressure dependent (the old P=V/T formula) and the lower the pressure the lower the boiling point. Theoretical boiling point of water in a vacuum is something on the order of -46 degrees.
