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 mutation in the sex cells of a parent can cause a new trait to appear in the parent 's offspring. 
Sentence2: cancer of a parent's cells can cause a new trait to appear in their offspring.
Example Output: appear

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: Echinoderms lack respiratory and excretory systems. 
Sentence2: echinoderms can't contract influenza.
Example Output:
Echinoderms