I recently spoke with Munroe about What If, xkcd, creativity, baseballs pitched at 90 percent of the speed of light ... and how, for him, The Lord of the Rings helped lead to it all. The conversation below is lightly edited and condensed.

First things first: Why did you create What If?

It actually started with a class. MIT has a weekend program where volunteers can teach classes to groups of high school students on any subject you want. I had a friend who was doing it, and it sounded really cool -- so I signed up to teach a class about energy, which I always thought was interesting, but which is a slippery idea to define. I was really getting into the nuts and bolts of what energy is, and it was a lot of fun -- but when I started to get into the normal lecture part of the class, it felt kind of dry, and I could tell the kids weren't super into it. And then we got to a part where I brought up an example -- I think it was Yoda in Star Wars. And they got really excited about that. And then they started throwing out more questions about different movies -- like, "When the Eye of Sauron exploded at the end of The Lord of the Rings, and knocked people over from this far away, can we tell how big a blast that was?" They got really excited about that -- and I had a lot more fun doing it than I did just teaching the regular material.

So I spent the second half of the class just solving problems like that in front of them. And then I was like, "That was really fun. I want to keep doing it."

So What If was basically a spin-off of the class?

That was where the idea came from. I actually wrote the first couple entries quite some time ago, based on questions students asked me in that class -- and then on another couple questions that my friends had asked. It was only recently that I finally managed to get around to starting it up as a blog.

The variety of the topics you tackle is incredibly broad. How do you figure out the best way to explain all these different, complicated subjects to people?

Part of what I'm doing is selectively looking for questions that I already know something interesting about. Or I've stumbled across a paper recently that was really cool, and now I'll keep an eye out for a question that will let me bring it up -- something I can use as a springboard. So in a conversation, someone might say, "Money doesn't grow on trees." Okay, well, what if money did grow on trees? Our economy would collapse. On the other hand, we would switch to a new currency. It's complicated.

What I like doing is finding the places in those questions where normal people -- or, people who have less spare time than I do -- think, "This is stupid," and stop. I think the really cool and compelling thing about math and physics is that it opens up entry to all these hypotheticals -- or at least, it gives you the language to talk about them. But at the same time, if a scenario is completely disconnected from reality, it's not all that interesting. So I like the questions that come back around to something in real life.