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: Polyps usually reproduce asexually.
Sentence2: Polyps usually reproduce several different ways.

Output: Polyps

Input: Consider Input: Sentence1: microscope is used to see small things by biologists by making them appear bigger.
Sentence2: Biologists use microscopes to study things that are too small to be seen with the unaided eye.
Output: biologists