I searched around and could not find anything that gave me a strait answer as to how they work. I put one in my car while i was running errands to keep some stuff from freezing (soda, wasnt able to put it anyplace warmer) and i notice that the packet wasnt warm at all. It was warm before i put it on the soda. I pick it up and play with it for a few seconds and its warm again.

So whats going on? whats the activator for them? I thought it was oxygen but i am starting to think its moisture in the air. The cold dry air must not be providing whatever they need to work

Any ideas?

iron powder, water, salt, activated charcoal, and wood fiber

activated by oxygen.

I'm pretty sure it's oxygen.  We use them all the time and there are a variety of situations where they seem to fade and you have to shake them up and reexpose the whole contents of the packet to the air.  If it crapped out when it was against your soda can it is probably because condensation on the can got the packet wet and essentially shut off the oxygen to the chemicals inside.

Depends on the type.  There is also a reusable variety that has a supersaturated salt solution in it; it releases heat as the contents crystallize.  I prefer these to the oxidization type since they can be reused by boiling them in water for a few minutes, then letting them cool slowly.

Oxygen, but it does need water.  I opened one once.  After a while it dried, and didn't warm up any more.  I put a little water in it and it immediately warmed right up.