how do lipos go up in smoke/ flames?

[Sammus]

They only explode if your stupid like people on those videos, and hook the terminals directly into a power socket or a car battery or stab them with a screwdriver or something stupid. I've never in my life seen a lipo explode, or even vent, under normal use, but I have seen many nicds and nimh's do this. with the pressure build up in a steel can they can have a big bang.