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: Finally, the fluid in the nephron reaches a collecting duct . 
Sentence2: fluid in the kidneys collect into ducts.

Output: fluid


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

Output: refraction


Input: Consider Input: Sentence1: Solar energy is clean energy . 
Sentence2: Solar light enters ecosystems.


Output: Solar