I do a stunt where I take some liquid nitrogen into my mouth and spew
it upward in a spectacular fountain. It is not really a "demonstration"
unless a test of the third property of Nitrogen (colorless, odorless
and...). I really do it because I *can* do it and because it has the
proper effect; very few of my students have seen it before. (There are
lots of other good, less dangerous, Leidenfrost effect demonstrations.)
I even work this one into my astronomy course when I talk about the
geysers on Triton.
Of course I go beyond the "Don't try this at home, kids" warning,
explaining to them some of the bad things that can happen if this is
not done right, and I don't elaborate with instructions on how to do it
right!