"Flammable" means "it can burn." Like a piece of wood. If you hold a match to it for a while, it will eventually start burning.
"Inflammable" means "capable of bursting into flame." Like a pool of gasoline: If a flame touches it, it will be a raging fire, instantly.
So these two words are not exactly interchangeable; there is a difference in meaning.
We don't know who it was that discovered water, but we're pretty sure that it wasn't a fish. -- Marshall McLuhan