Entropy

Entropy: "a thermodynamic quantity representing the amount of energy in a system that is no longer available for doing mechanical work; entropy increases as matter and energy in the universe degrade to an ultimate state of inert uniformity."

http://www.multivax.com/last_question.html

Easily one of my favorite short stories from Isaac Asimov. The first time I read it, the ending shocked the hell out of me. If you haven't read it, definitely take 10 minutes and do so.

Happy 2009.