What is funny in Inflammable

In one episode of Simpsons there is joke when fire catches because a foreign citizen read "inflammable" on a gas bottle, and said it is all right.

The joke arises from the fact that "inflammable" and "flammable" are synonymous (that is, they have the same meaning.) Normally the prefix "in" makes the root word have the opposite meaning, as in: "justice/injustice", "sensitive/insensitive", "eligible/ineligible." etc.

Mistaking this inversion makes the non native English speaker think that the "inflammable" gas is the opposite of flammable, which is to say "not flammable." A mistake that could have unfortunate consequences.