Aren't you interesting?

The abbreviation n't is a very curious one; it saves the writer a mere white space and two-thirds of an o and so isn't what one could call efficient (except in the occasional call like won't, obviously). The speaker is saved a few milliseconds as they run the two words together, but the abbreviation really affects speech.

I'll start with an obvious example;

"Don't do that!"

"Do not do that!"

The first sounds playful, happy, as though you're asking someone to stop tickling you; the second is your mother warning you against poking your brother with a 1:32 model of the Bat Mobile.

Including not into a sentence adds weight to the message: "We are not going to the park," "You shall not pass," and "I have not got any hash." You read the word not with a certain, aggressive inflection, and really I should have italicized each one.

However, there are some cases where the word not just doesn't fit in a sentence where n't does; the abbreviation makes the sentence sensible, where as not makes the sentence clunky: "Aren't you cold?" as opposed to "Are not you cold?"

"Is not it Thursday today?

"Have not you got the keys?"

The sentences just don't seem right, despite making complete grammatical sense, and you would certainly be suspicious of someone who talked like that. I guess it's just because of how we're used to speaking, but it's certainly a curiosity, ain't it?