Imagine a square, sitting on a piece of paper and talking to his friend, Ms. Circle. As their universe is infinitely thin, Mr. Square would only see a line segment of Ms. Circle, even though that’s not the shape she is. A two dimensional object cannot “see” two dimensions– only one. Likewise, three dimensional humans cannot actually see three dimensions, only two dimensions at a time.
Here’s where it starts to get cool. If Mr. Square was falsely convicted of grand larceny and had to go to Square Penitentiary, we could say “oh, you can easily escape! Just hop vertically out of the square!” Of course, he wouldn’t know how to do that. The third dimension is beyond his comprehension of the world.
Likewise, if a human was stuck in a jail cell, there would have to be a fourth dimension we could escape through. There is *mathematically a direction that we simply can’t point to because we cannot perceive it.* For all we know, fourth dimensional beings are walking all around us, watching our every move.
Sleep well, kids.