Mention a change which is always desirable, undesirable and periodic.
- Desirable : Changes that are useful to man are desirable changes, e.g. change of milk into curd.
- Undesirable : Change that brings about destruction is an undesirable change, e.g. floods and epidemics are undesirable changes.
- Periodic : Changes that are repeated at regular intervals of time are called periodic changes, e.g. change of day and night.