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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.

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Hugely related and may even be a duplicate: english.stackexchange.com/questions/1578/… – St John of the Cross Mar 29 '13 at 20:00
up vote 5 down vote accepted

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.

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It's the in₂- prefix that means not. The in₁- prefix means, um, in. So inflame means 'burst into flame', and thus inflammable means 'capable of bursting into flame'. There's lots of other in₁- words: inform, intense, intent, injure, intern, income, ... – John Lawler Mar 29 '13 at 21:04
This brings to mind a fondly remembered passage of Strunk and White - 'Flammable. An oddity, chiefly useful in saving lives. The common word meaning "combustible" is inflammable. But some people are thrown off by the in- and think inflammable means "not combustible." For this reason, trucks carrying gasoline or explosives are now marked FLAMMABLE. Unless you are operating such a truck and hence are concerned with the safety of children and illiterates, use inflammable.' – Jeff Yoak Mar 29 '13 at 23:39

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