If that blew your mind then this will melt your brain:
2D man can see only one side of a square at any given time. Actually it's possible for him to see two sides and a vertex if the square is being rotated. He'd see a front on view of one side, then two foreshortened edges with a moving vertex as the two sides change in length until the next side comes fully into view. 2D man is now viewing one side straight on perpindicular to himself at full length. The rotating box will appear to him for one instant as a line with two end points, we can see this by simply imagining ourselves viewing the square edge wise as it rotates.
Here's the insane part: from our higher third dimension we can see all four sides of the square including what might be inside it. Something the lower, 2D man could never do. Similarly when a viewer from a higher 4th dimension looks 'down' on a cube in our 3-dimensional world he sees all six sides of the cube simultaneously. He can even see inside.