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.

Example Input: Sentence1: Climate refers to weather over a long period of time. 
Sentence2: weather is generally described in terms of temperature and moisture.
Example Output: long

Example 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.
Example Output: species

Example Input: Sentence1: the mass of a planet causes the pull of gravity on that planet. 
Sentence2: Jupiter has a greater gravitational pull than earth.
Example Output:
gravity