Allusion!

Allusion is possibly my favorite aspect of literature.  I didn't learn what allusion was until my senior year of high school, when I took an independent study called Short Story (an independent study is a class that is just you and the teacher, and instead of going to a class period every day, you just touch base once a week to exchange assignments and such).

So what is allusion?  Here's what the internet says: "a passing or casual reference; an incidental mention of something, either directly or by implication: an allusion to Shakespeare" (Dictionary.com).

In Harry Potter, Harry gets a scar on his forehead that imbues him with some special power that protects him from the bad guys.  In The Wizard of Oz, Glinda the good witch kisses Dorothy on the forehead, and Dorothy is actually protected by the kiss (it may be more obvious in the Wizard of Oz books, but whenever people are roughed up by the bad guys, Dorothy gets by pretty unscathed).

So, the allusion is that a mark on the forehead grants protection from evil.

So allusions are basically references to other things, and they tend to be subtle, almost sneaked in there.  That means that in order for an allusion to work, the reader must be familiar with the original work being alluded to.

Once you understand the concept of the "star-crossed lovers" from Shakespeare's Romeo and Juliet, you'll notice the allusion whenever two young people from opposing parties fall in love, such as the film Underworld or the musical West Side Story or the film Titanic.

For me, the less likely one is to understand the allusion, the more exciting it becomes.  Sometimes I catch allusions because I just so happen to know what is being referenced, though it isn't particularly popular at all.  I enjoy allusions because it means that authors also know/enjoy the original work, and that they put thought and effort into their work.