What is the surprising thing that happens in a superconductor?

With almost all substances . . .

-- when you cool them, their electrical resistance decreases.
-- If you make them even colder, their resistance decreases more.
-- If you make them even colder, their resistance decreases more.
-- If you make them even colder, their resistance decreases more.

-- If you keep making them colder, their resistance keeps decreasing,
but it never completely disappears, no matter how cold you make them.

But with a few surprising substances, called 'superconductors' . . .

-- when you cool them, their electrical resistance decreases.
-- If you make them even colder, their resistance decreases more.
-- If you make them even colder, their resistance decreases more.
-- If you make them even colder, their resistance decreases more.

-- If you keep making them colder, then suddenly, at some magic 
temperature, their resistance COMPLETELY disappears.  It doesn't 
just become small, and it doesn't just become too small to measure.  
It becomes literally totally and absolutely ZERO.  

If you start a current flowing in a superconducting wire, for example, 
you can connect  the ends of the wire together, and the current keeps 
flowing around and around in it, for months or years.  As long as you 
keep the loop cold enough, the current never decreases, because
the superconducting wire has totally ZERO resistance.

Did somebody say "What's this good for ?  What can you do with it ?"

1).  Every CT-scan machine and every MRI machine needs many 
powerful magnets to do its thing.  They are all electromagnets, with 
coils of superconducting wire, enclosed in containers full of liquid helium.  
Yes, it's complicated and expensive.  But it turns out to be simpler and 
cheaper than using regular electromagnets, with coils of regular plain
old copper wire, AND the big power supplies that would be needed
to keep them going.

2).  Resistance in wire means that when current flows through it,
energy is lost.  The long cables from the power-generating station
to your house have resistance, so energy is lost on the way from the
generating station to your house.  That lost energy is energy that the
electric company can't sell, because they can't deliver it to customers.

There are plans to build superconducting cables to carry electric power
from the producers to the customers.  The cables will be hollow pipes,
with liquid helium or liquid hydrogen inside to keep them cold, and 
something on the outside to insulate them from the warmth outside.
Yes, they'll be complicated and expensive.  But they'll have ZERO
resistance, so NO energy will be lost on its way from the generating 
stations to the customers.  The power companies think they can
build superconducting 'transmission lines' that will cost less than
the energy that's being lost now, with regular cables.