Plot Device

Some of the best known plot devices are easily recognizable. For example, The One Ring in J. R. R. Tolkien's Lord of the Rings, or Zeus' Lightning bolt in Percy Jackson & the Olympians. According to Wikipedia, a plot device is an object or character in a story whose only purpose is to advance the plot of the story, or alternatively to overcome some difficulty in the plot. To me, a great plot device plants a seed that works within the rest of the plot; it gives the character(s) motivation, and helps move the story forward. And I think they make a story more interesting. Can you think of some other well-known plot devices?

Below is a really fun video by Seth Worley.