What is the difference between crystalline and amorphous solids?
A typical crystalline solid is held together by strong intermolecular forces (IMFs), and can usually be found as unit cells repeated in 3 dimensions (i.e. lattice structures).
An amorphous solid like glass or plastics has a less patterned structure, but is rigid nonetheless, and also has strong IMFs.
Because of those strong IMFs, many solids have rigid structures. By definition, rigid structure
You can find more about properties of solids here.