While it definitely lacks the low-budget charm of this nearly identical classroom presentation, this demonstration of liquid nitrogen's explosive potential does have two major things going for it. One: more ping pong balls. Two: A high-speed camera, for slowmo playback goodness.
As hosts Mauricio and David explain in the video, a mole of nitrogen in its gaseous phase takes up WAY more volume than it does in its liquid phase, so confining, say, a cup and a half of liquid N2 to a container that can't handle a rapid spike in pressure has some pretty explosive consequences.
You'll want to skip to around the 1:15 mark for the business.