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: Rivers, rainwater, well water and most lakes contain fresh water. 
Sentence2: canyons are formed by rainwater.

Ex Output:
rainwater


Ex Input:
Sentence1: Gravity is a force that pulls on things. 
Sentence2: A force that pulls on things pulls objects towards planets.

Ex Output:
force


Ex Input:
Sentence1: Polyps usually reproduce asexually. 
Sentence2: Coral polyps are united into colonies.

Ex Output:

Polyps