Lanterns
With a prime lens at its widest aperture setting, like a 50mm f/1.8, only a very narrow slice of what is in front of you is in focus. The farther away from the subject you're focused on, the thicker that slice gets. If you're focusing on something 3 feet in front of you, the slice might only be 1 inch thick. Focus on something 8 feet away, it becomes a slice 10 inches thick. The numbers aren't perfect, just examples used to illustrate the principle.
The important aspect of this is that you can have one thing in your picture perfectly in focus, and have everything else in the picture out of focus. This technique can be used to awesome effect :D
Notice how only the middle lantern is in focus?
