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: mineral graphite is used for making pencil lead. 
Sentence2: Diamond and graphite are made of carbon.

Ex Output:
graphite


Ex Input:
Sentence1: Cells are the microscopic building blocks of organisms. 
Sentence2: proteins are the building blocks of organisms.

Ex Output:
organisms


Ex Input:
Sentence1: Echinoderms lack a centralized nervous system. 
Sentence2: From animal experiments, it seems to be a central nervous system toxin.

Ex Output:

centralized