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Ceiling
Interior Surface
Remember, 'ceiling' refers to the interior upper surface of a room, not any upper surface outside. 
Not the Roof
'Ceiling' is the underpart of a roof and is found indoors, but it's not the same as 'roof', which is external. 
Decorative Aspect
Ceilings can have decorative elements, like moldings or painted designs, which add to a room's character. 