"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.
"An idealist is one who, on noticing that a rose smells better than a cabbage, concludes that it will also make better soup." - H.L. Mencken