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.

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

Ex Output:
refraction


Ex Input:
Sentence1: when a hurricane moves over land , that hurricane will decrease in strength. 
Sentence2: Hurricanes form over the warm tropical water in ocean and die down when they move over land.

Ex Output:
land


Ex Input:
Sentence1: cutting down trees has a negative impact on an ecosystem. 
Sentence2: Cutting down trees has a negative impact on the homes of animals  in natural habitats.

Ex Output:

cutting