The board in the section.
Under the load, its end is lowered. The upper part is slightly lengthened. The material occurs in the material. The inside is shortened. The material is compressed. The middle layer of the material does not change in size, its reaction is neutral. In fact, the board behaves like a console that, leaving the wall, supports the balcony, or as a bracket fixed on the wall and supports the book shelf. This is the beam — the main element of mechanical structures and buildings. The more the material from which it consists, it resists stretching and compression, the less the beam deforms. So, it has become well behaved in both cases. But the stone resists compression well, but easily breaks down with stretching. Almost the same thing happens with concrete. Reinforced concrete was invented to overcome this lack.