By Oliver Baker for NCSA -- CHAMPAIGN, IL--As high-school chemistry may have taught you, it's easy to calculate the odds that a molecule you've just inhaled came from the dying breath of Julius Caesar. But imagine you need to account for the molecule's experiences in the meantime. Say you have to weigh the chances of each chemical transformation it might have undergone -- as it ascended from ancient Rome, drifted through the dirt and droplets of clouds, and endured temperatures, solar rays, and collisions with gaseous chemicals in changing combinations. Now for extra credit, explain how the global distribution of gases shifts and evolves. Include all life, pollution, inanimate matter, and climate change.