Metals are said to be shiny. Why do metals generally appear to be dull? How can their brightness be restored?

Solution:

When metals are exposed to air for an extended period of time, a thin layer of oxide, carbonate, or sulphide accumulates on their surface due to the gradual action of various gases.

Brightness can be restored by rubbing the dull surface with sand paper, then removing the outer layer, resulting in a shining metal object.