During a recent discussion of Class I Division 1 requirements, I explained it this way:
Class I Division 1 means I can pour gasoline on the machine and it will keep running without igniting it.
Class I Division 2 means that I can pour gasoline on the machine and I can shut it down before it ignites, and when it is off it won't ignite the gasoline.
I know I omitted the group here but that complicates the explanation. Anyone who's ever tried to re-start a charcoal grill after re-lighter fluiding it or used gas to start burning leaves or scrap, knows from their unsuccessful (or maybe successful) attempt at winning a Darwin award, how easy it is to start the fire/explosion.
If anyone has better analogies, feel free. When you specify equipment for a classified location, the accountants become apoplectic and need a good answer where they walk away understanding, not thinking that maybe they're being techno-bs'd.