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.

Ex Input:
Sentence1: Bird eggs have hard, calcium carbonate shells. 
Sentence2: The reproductive cells of a bird is a hard, calcium carbonate shell.

Ex Output:
calcium


Ex Input:
Sentence1: Polyps usually reproduce asexually. 
Sentence2: Polyps usually reproduce several different ways.

Ex Output:
Polyps


Ex Input:
Sentence1: Mollusks have a circulatory system with one or two hearts that pump blood. 
Sentence2: Mollusks have hearts that circulate blood through the body.

Ex Output:

Mollusks