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: 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: All photos are digital images. 
Sentence2: satellites are used to take photos from space.

Output: photos


Input: Consider Input: Sentence1: Octopuses spray a cloud of ink at their predators. 
Sentence2: Octopuses spray a cloud for hiding from predators.


Output: cloud