Re: how do all the particles in liquid nitrogen get so cold?

Liquid nitrogen is cold for the same reason that ice cubes are cold,
someone has gone through the trouble of cooling it down.  
Nitrogen freezes at a very low temperature (-350 degrees F) 
and becomes a gas at (-320 degrees F).  So even if the temperature 
outside was -100 degrees F nitrogen would still be a gas.  In fact, 
most of air (about 80% of it) is made of nitrogen.  It's estimated
that the earth's atmosphere contains 4000 trillion tons of nitrogen!

Nitrogen is not cold by itself, in the same sense that water isn't
inherently warm or cold.  In order to produce liquid nitrogen, you
need to cool nitrogen gas down below -320 degrees F (but not below
-350 degrees F or it will be frozen).  This must be done with
a refrigerator of some sort, that can reach this low of a temperature.

The reason that liquid nitrogen is used so often, is that nitrogen
is relatively inert, which is a fancy way of saying that it doesn't
like to interact with other elements.  Because of this, you don't have
to worry about side effects of dipping shrimp in a vat of liquid
nitrogen to freeze it.