Jan,
OK, I guess I should have included this in the first post.  The reason that
distilled water is said not to "boil" is that it does not make bubbles
easily then.  This can happen with clean tap water too but is less likely.
In order to make the bubbles for boiling first the water must be 100C (212F)
and it must have something to form a bubble around.  To form the bubbles you
need to break the surface tension of the water.  You are more likely to get
the "explosion" (for lack of a better term) in the microwave in a glass
dish, cup or bowl.  In a regular metal saucepan, there is enough roughness
on the bottom of the saucepan that the water can form the bubbles and some
of the energy is released.  Also, the water heats more unevenly on the stove
than in a microwave.  When one part is hotter than another, it is more
likely to have bubbles.
On a very smooth surface, like glass, there is nothing for the bubbles to
form on if there are no impurities and the water appears not to boil.  It
becomes superheated.  This is more likely in a microwave because the water
does not mix around as much when it heats.  When something is added, like
salt or a spoon, all of a sudden there are thousands of particles to form
bubbles and in a rush of energy, the whole thing boils up all of the bubbles
that it had been saving.  This looks like an "explosion" and it appears that
the water has become hotter all of a sudden.
This impacts us because we are accustomed to looking for bubbles to know
that water had reached 100C and if there are no bubbles, we assume it is not
hot enough.  When a little bit of something is added the water which must be
over 100C, the whole thing can boil over and scald someone.

Linda Pohl, IBCLC, former (maybe current?) nerd

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