The flexibility of the internal structures of solids, both crystalline and amorphous, is a result of the freedom of the displacement
of their elements, without disturbance of the continuity of the structure as a whole. This article discusses peculiarities
of solids with flexible structures. The effects of flexibility in the thermal reactions of the creation of new compounds in
the internal structure of solids, and the phase transitions, are considered. Flexibility is indispensable for random amorphous
structure formation. The glass transition effect is a consequence of the high flexibility of the structure of glasses.