Basically, the best way to look at solids is to see them as a spring. Yes! A simple spring represents how solids work at the molecular level. How? Well, a spring compresses itself when a load pushes it, and it stretches itself when a force pulls it.
According to Robert Hooke, this is exactly what happens with the material at the molecular level; the chemical bonds which hold the atoms together are very strong and stiff, which allows the molecules to compress or stretch themselves without easily breaking. Of course, we do not see this molecular movement unless we are using a huge microscope that can allow us to see the behavior of atoms and molecules. However, the fact that it is not visible to the naked human eye, does not mean it is not happening.
With Hooke’s Law in mind, we can evidently realize that when a material or a structure is resisting a load of x force, it is actually pushing back at it with an opposite and equal force; if it were not pushing back at it, the material will clearly deflect in the direction of the force. This load can either pull or push a material or structure.
Solid materials change their shape by stretching or compressing themselves, every time a mechanical force is applied to them. Most of the time, this change in shape will be invisible to the naked human eye, but it is still happening at the molecular scale. Therefore, because the solid changes in shape (the atoms and molecules are reorganized), it is enabled to push back at it (just as a spring pushes back). However, when the force of the load is higher than the force the material is able to push back or withstand, we see a deflection or deformation in the material or structure.