Most of us are accustomed to watching movies in 2D; even though characters on the screen appear to have depth and texture, the image is actually flat.

But when we put on 3D glasses, we see a world that we could walk in. We can imagine existing in such a three-dimensional world because we actually live in one. But for someone who's only known life in two dimensions, the third dimension would be impossible to comprehend.

And that, according to many researchers, is the reason we can't see the fourth dimension, or any other dimension beyond that. Physicists work under the assumption that there are at least 10 dimensions, but the majority of us will never "see" them. Because we only know life in 3D, our brains don't understand how to look for anything more.