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: the mass of a planet causes the pull of gravity on that planet. 
Sentence2: the mass of a planet is what causes things to have weight.

Output: mass


Input: Consider Input: Sentence1: microscope is used to see small things by biologists by making them appear bigger. 
Sentence2: Something invented by Leeuwenhoek is used to see small things by biologists by making them appear bigger.

Output: appear


Input: Consider Input: Sentence1: Biodiversity increases the productivity and stability of ecosystems. 
Sentence2: Marine ecosystems also act as stabilizers of global systems especially our climate.


Output: ecosystems