The ends of the torn steel strip do not grow back together when they are attached to each other.

The ends of the torn steel strip do not grow back together when they are attached to each other. Why, by heating two strips of steel red-hot, the blacksmith can simply connect them with hammer blows?

To connect two parts of the strip together, you need to attach them to each other so that a large number of molecules are at a distance from each other, at which the forces of intermolecular attraction begin to act. The surface of the steel is uneven (in comparison with the size of the molecules); this unevenness can be eliminated only when the steel becomes ductile in the hot state. This is what the blacksmith does with hammer blows.