Because liquid nitrogen is cold. Very cold.

http://www.popsci.com/popsci/how2/article/0,20967,1022179,00.html
http://www.popsci.com/popsci/how2/article/0,20967,1022179,00.html
http://www.popsci.com/popsci/how2/article/0,20967,1022179,00.html