ah~! The principal of 3, barry mentioned that today, lol. XD
But yeah, squares, circles and triangles are the key shapes that people use for design, and they also can communicate feelings, too.
Like, say... they change the mood of a scene, say the traingles were skinny and tall and formed a sharp spike. This could be the setting for something dark, like if you fell in a hole and... there were spikes at the bottom. >_> Ew... what did I do? XD
Well anyway, we manipulate those shapes the most to communicate ideas and come up with great moods and settings. (even characters)