We think of gasoline, lighter fluid and hydrogen as dangerous flammable substances (in the ordinary, not philosophical, sense). But in the ordinary course of things, when they burn, they do so because of oxygen. So it seems more reasonable to think of oxygen as the dangerous inflamer here. This is of course a very standard example in philosophy of causation: we don't normally think of oxygen as the cause of a fire, but we could just as well, except for pragmatic stuff. I just didn't realize until recently how great the example is. Having oxygen about is having a fire about to happen. Thinking about this also makes it clear just how precarious our existence is, dependent on such a highly dangerous chemical as it is.