How is depression different from feeling sad?

I think being sad can last for a couple of minutes/ hours
But being depressed can last for days/months/years.. makes you shut out of the world, makes you uninterested in things, makes you have suicidal thoughts and other stuffs.
They’re both similar..(my opinion)

Smoking damages the respiratory system by inhalation of a foreign substance, that is absorbed and residue is left behind on our lungs sponge like substance. the gas that is inhaled also increase your blood pressure and dilates your blood vessels. also the process of inhaling and exhaling the smoke it causes irritation to the throat and mouth. smoking can cause lung cancer and other respiratory and circulatory diseases. 

There is tumerous cancer, kawaski disease and more.