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.

Input: Consider 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.

Output: species


Input: Consider Input: Sentence1: Fish are the first known true chordates. 
Sentence2: Fish are the first known animal with a backbone.

Output: Fish


Input: Consider Input: Sentence1: forming coal requires decaying vegetation. 
Sentence2: decaying vegetation forms energy.


Output: decaying