In the 1880's, a German psychologist named Herman Ebbinghaus wanted to understand how memories decayed over time, and he especially wanted to know how long the process took.
What started more than four years ago as a banking and sovereign-debt crisis has decayed into a growth crisis that is now enveloping the three biggest economies.
Fossil fuels ultimately derive from decayed plants and animals, which contain a variety of carbon that has a different atomic mass than most carbon in our atmosphere.