thing vs necessity

thing and necessity both are nouns.

Word	Noun	Adjective	Verb	Adverb
thing	Yes	No	No	No
necessity	Yes	No	No	No

As nouns, necessity is a hyponym of thing; that is, necessity is a word with a more specific, narrower meaning than thing:

thing (noun)	necessity (noun)
a separate and self-contained entity	anything indispensable
an action	the condition of being essential or indispensable
an artifact
an entity that is not named specifically
any attribute or quality considered as having its own existence
a special abstraction
a special objective
a vaguely specified concern
a statement regarded as an object
an event
a persistent illogical feeling of desire or aversion
a special situation