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: propulsion is used for flying by a spacecraft. 
Sentence2: Jets are used for flying by spacecraft.
Example Output: flying

Example Input: Sentence1: a rainbow is formed by refraction of light by separating light into all different colors. 
Sentence2: Rainbows form when raindrops refract sunlight.
Example Output: refraction

Example Input: Sentence1: Tulips perform best in a sandy, organically enriched soil. 
Sentence2: Earthworms are important deposit feeders that help form and add organic material to soil.
Example Output:
organically