Shade Smooth vs. edges Here is a strange thing:
it's a polyhedron and it's shade is flat:

i want to smoothen it's faces, so i use &quot;Shade Smooth&quot;:

weird things happend with edges - no good.
Than i separate polyhedron's faces:

and here it is - result that i want. Than i join back polyhedron's faces:

and it's edges don't retein their previous &quot;weird&quot; state, they stay the way that i want them to be.
Why is it happens, and is there a shorter way to get result that i want?
By splitting and re-joining the faces, you're creating double edges.
That improves shading on sharp edges (since the edges are no longer continuous). 

You can achieve the same by adding the Edge Split Modifier.



Or by selecting all vertices and using Edge > Split