A brief introduction to the concept of temperature.
A link to an article on negative Kelvin temperatures: http://www.sciencedaily.com/releases/2013/01/130104143516.htm
UPDATE [Jan 2014] : It seems that negative Kelvin temperatures may not be possible after all. See this article: http://www.sciencedaily.com/releases/2013/12/131220113639.htm
