Lately I've been re-reading Strunk and White's The Elements of Style, which is, as you know, the best book every written about writing.
This part is one of my favourites:
"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 the the safety of children and illiterates, use inflammable."
The reasons I like this:
1. It's witty.
2. I will now examine trucks closely.
3. Because it makes me contemplate just how much (a lot) of my life has been spent concerning myself with the safety of children and illiterates.