Sentence:  These balls are moving in great circles and ellipses , and <WSD>are</WSD> of course , the electrons , the particles of negative electricity which by their action create the forces that tie this atom of calcium to the neighboring atoms of oxygen and make up the solid structure of my finger bone . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)