When doing research, students will regularly ask me, "Why do I need to find multiple sources? I can get all the information I need from this [article, book, website]." I always explain that if you only use one source, you don't know what you're not getting from other sources in terms of information, ideas, viewpoints, etc. Students are, generally speaking, convinced by my line of argument, but I'd been looking for a good analogy to explain it (I love a good analogy).
And I think I may have come up with one. Technically, it's not my analogy, but I'm pretty happy with the presentation, so I thought I'd share! I took the story about the elephant and the blind men, used flickrcc.bluemountains.net (my favorite source for Creative Commons images) to find images to illustrate the story, and put together a quick slideshow:
I'll be trying it out tomorrow with a class coming in to find sources for a persuasive essay--a perfect opportunity to talk about the importance of getting multiple viewpoints.