Updated Jan. 1, 2013: I've added a postscript to this post. You can find it at the bottom of this page.
Look at yourself. Right now.
You are muscle,skin, bone, brain, blood, warmed by energy, and all of you, every cell, even the subsets of those cells, all trillions and trillions of them, are going to tire, waste and depart. In 10 years almost every bit of you will have been replaced by new bits.