I've been reading up on several CO2 methods and it seems that most recommend some sort of slow-introduction of CO2 such that the animal goes to sleep before dying. Well, wouldn't the same be accomplished if the animal were just in a sealed container? They would slowly run out of oxygen, pass out, and if left in there long enough, die. What is the reason that active introduction of CO2 is necessary? Wouldn't time and a sufficiently leak-proof container do the trick just as well?