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: 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: Some viruses live in a dormant state inside the body. 
Sentence2: a virus can live in the brain.
Example Output: viruses

Example Input: Sentence1: intense heat has a negative impact on an organism. 
Sentence2: intense heat has a negative impact on plants.
Example Output:
heat