Liquid nitrogen can be produced industrially in large quantities by fractional distillation of liquid air and is often referred to by the abbreviation, LN2.  It is a cryogenic fluid which is potentially capable of causing instant frostbite on contact with living tissue, as reportedly happened on one year's mission. 
Spacesim's Uses of Liquid Nitrogen
Spacesim's EEP makes good use of LN2 as it's Pit Demo demonstration for Neptune. The presentation involves freezing flowers, and showing how brittle they become by smashing them on a desk. Additionally, marshmallows frozen in the substance are distributed to the children attending the EEP.
LN2 is arguably Spacesim's most used selling point with reference to successful PR stunts. Frozen Marshmallows, Smoking Bottles of Doom, and Exploding Bottles of Doom are always widely appreciated by Spacesim's audiences.
Smoking Bottle of Doom
The Smoking Bottle of Doom is an LN2 demonstration involving pouring the liquid into any available pop bottle (541mL bottles recommended), though Sprite bottles are favorite because they smell nicer.<ref>Stefan's Personal Opinion</ref> The demonstration was first created at an EEP Training Session by Stefan De Young, and has since been an integral part of Spacesim's LN2 demo arsenal.
Exploding Bottle of Doom
The Exploding Bottle of Doom demo created by Dr. Magwood, and first used at the 2006 Week of Hype, takes advantage of liquid nitrogen's high volatility at SATP. The demo involves half filling a 541mL pop bottle with LN2, capping it, and placing the bottle in a large container of lukewarm water. Then the presenter will run away from the container as quickly as possible before the LN2 expands enough to explode the bottle.
Warning: This demo is only to be done under the Admiral's Supervision, and in an open space.
Spacesim obtains all of its liquid nitrogen from BOC Gases for the small fee of 2$/L.
 "Nitrogen." Wikipedia, The Free Encyclopedia. 9 May 2007, 23:59 UTC. Wikimedia Foundation, Inc. 10 May 2007 <http://en.wikipedia.org/w/index.php?title=Nitrogen&oldid=129680459>.
 Stefan's recollection of Dr. Magwood's story