Experiments with deformation of bodies. During compression, the distance between the molecules decreases and forces of mutual repulsion arise, which leads to the emergence of elastic forces that tend to return the body to its original shape. When stretched, on the contrary, the forces of mutual attraction tend to compress the body back.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.