Sarcasm is like a truth-lie

You say something you don't literally mean, and the hearer only understands if they get that you're insincere. The ability to recognize sarcasm is an essential skill to function in a modern society that thrives on irony.

Entire phrases have lost their literal meaning because they are so frequently used with a sneer. For example, "big deal", or "tell it to someone who cares," and "aren't you special" means you aren't.