It turns out, that the fact, that hard water will readily conduct
electricity, has been used since at least the 1940s to cook eggs!

I have a neato ceramic Hankscraft Art Deco egg cooker that uses two
concentric circles of wire.  The two circles do not touch each other,
but are laid in the bottom well of the cooker about 1/4 inch apart.
These conductors are connected to a standard power cord for utilizing
normal household current.

To cook the eggs, you place a couple tablespoons of water in the bottom
of the cooker, add 4 eggs, cover and plug in the unit.

The current flowing through the water between the conductors heats the
water until it boils , the resultant steam cooks the eggs.  When all
the water has boiled off, the circuit is opened, not more current is
flowing, no more cooking. The amount of water used at the beginning
will necessarily affect the length of time the eggs are cooked.  In
the event the water is not naturally hard enough, you add a pinch of
salt.  Ain't science wonderful ?

Tom Lear
San Francisco, Ca.