instruction:
Generate an overlapping word between the given two sentences. When you find the overlapping words, they don't have to match exactly, e.g., "survival" and "survive" are valid overlapping words. Little words like "the" or "of" don't count! You must generate significant words which are not the stop words.
question:
Sentence1: Bird eggs have hard, calcium carbonate shells. 
Sentence2: The reproductive cells of a bird is a hard, calcium carbonate shell.
answer:
calcium


question:
Sentence1: hearing is used for sensing sound by animals. 
Sentence2: functioning ears are used for sensing sound by animals.
answer:
animals


question:
Sentence1: wiring requires an electrical conductor. 
Sentence2: Wiring requires copper or gold.
answer:

requires