Does half-life mean something can never completely decay? Caffeine has a half-life of approximately six hours. I understand this to mean that every six hours, the amount of caffeine in the body is half of what it was six hours prior. Does that mean that caffeine never completely leaves the body? It just keeps reducing to half after a fixed time ad infintum.
I guess a broader statement of my question is: When calculating the half life of a thing, is it true that it will never reach zero? And how is this used/reconciled in a more practical setting like when calculating the amount of caffeine in the body?
 A: There is always a whole number of molecules, and at some point in time there will be zero molecules left. With time, it becomes increasingly likely that all the caffeine molecules have completely disappeared. 
However, in chemistry the "concentration" is the mean, or expected, amount of molecules. This is not a whole number. This is much like the way the expected roll of a dice is $3\frac{1}{2}$ even though there is no such number on the dice - the average of your dice rolls over many rolls will be $3\frac{1}{2}$.
This expected number gets smaller and smaller but never reaches zero.
Perhaps this simulation works on you computer.
A: For half-life to make sense, the assumption that the metabolism that breaks down caffeine at a speed proportional the total amount in the body, and not on concentration or other things. Then exponential decay holds. This assumption doesn't exactly hold in practice, and the exponential decay is only approximate.
And of course, the caffeine is not a continuous quantity, therefore at some point we are talking about fraction of a molecule. Continuous dynamics is a good approximation for the number of caffeine molecules, but eventually they will break down completely.
See also http://en.wikipedia.org/wiki/Half-life for more information.
A: Any chemical or radioactivity has a level below which it cannot be detected, so you are safe if you are drinking coffee.
Meanwhile, half-lives are an approximation that works very well when the number of particles involved is essentially uncountable by us. If you get down to ten atoms remaining that might decay, there is nothing dangerous going on. And I just made up the number ten, an actual physicist could give specifics about any such issue that worries you.
