> Did you make sure that the outside of the cup wasn't
wet? This often happens when filling a cup with water.
Assuming it was dry, that sounds cool.

This is a classic physics experiment in school. It's got
nothing to do with the outside being wet: The boiling of
the water draws the heat energy from the flame, so
that this heat doesn't ignite the paper. I'm not sure it
would go equally well with a plastic cup, though.

Travholt.