What causes the weird shading when you smooth shade a cube?

Support Lighting and Rendering
1

Everyone knows the infamous ugly shading when you smooth shade a cube (or other sharp structure):

enter image description here

My question is not how to fix it. My question is what causes the cube to be like this? I know “the normals are averaged”, but what exactly it means? Why doesn’t it look like a super smooth sphere, for example? What causes these separations between bright and dark sides?

enter image description here

2

It is just already what you know, it averages the normals to smooth the lightning.
If you want faces smoothed only within specific angles you can activate “Auto smooth”

grafik