Sentence:  Scientists assume that cholesterol ( from the Greek chole , meaning bile , and sterios , meaning solid ) <WSD>is</WSD> somehow necessary for the formation of brain cells , since it accounts for about 2 % of the brain 's total solid weight . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)