A factory can run without electricity and with coal power or with any other power sources. Chimney is not mandatory for a factory. File are also not mandatory component for factory. Sellers are required but not mandatory. The most important component for factory is workers. If workers will be not there then factory will be shutdown and will not called factory without workers.

River can be without Delta , Tributaries , Boats and Fishes. A river can be called river without Banks. So banks are the mandatory for a river.

A camera can be without Reels, Stand, Photograph and Flash. A camera is no use without the lens. So A camera cannot use without lens.

A bulb can be without Light , Glass, Current and argon. A bulb always have a filament.

A Pen cannot be without Nib. A Pen can be without Tube, Ink, Holder and Cap. So a pen have always the mandatory component is NIB.

A Dead can have bones but No movement.

A man can be without Teeth, Feet, Eyes and Hands but can't be without heart.

Switch, Regulator, Wire and Current is the component for fan but not mandatory. Blades are the mandatory component for the Fan.

Almost all animals have Eyes, Four legs, Tails and Horns. So we can say that all animal have the almost same feature and instincts.

Sugar, Fats and Water can be the part of Milk. But calcium should be the mandatory part of Milk.