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: Populations Populations are subsets of a species. 
Sentence2: Gene flow occurs when individuals move into or out of a subset of a species.

Ex Output:
species


Ex Input:
Sentence1: Mad Cow disease is a concern. 
Sentence2: pathogens can cause Mad Cow.

Ex Output:
Cow


Ex Input:
Sentence1: Menstruation begins in puberty and ends with menopause . 
Sentence2: Menopause occurs when something that begins in puberty stops occurring.

Ex Output:

begins