The shell, that looks as thought it were solid, is actually porous. That is, there are thousands of tiny holes in it, so small that it is impossible to see them. At the large end of the egg is an air pocket, filled with oxygen. A fertile egg is so delicate that it can be ruined by small mistakes. Sometimes a perfectly good egg won't hatch because it has been turned wrong end up, or shaken.
Mother knows best.
A mother hen reaches underneath her body and turns her eggs frequently with her beak. This is because the egg yolk tends to float toward the shell. When the egg lies too long in one position, the white separates and allows the yolk to float through, forcing the egg cell against the shell and killing it. The hen avoids this by changing the position of the eggs. She keep them warm by spreading her feathers over them.