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: Many birds perform elaborate courtship dances. 
Sentence2: Many birds perform dances before they mate.

Ex Output:
Many


Ex Input:
Sentence1: triceps contracting causes a person 's elbow to straighten. 
Sentence2: muscles contracting cause a person's elbow to straighten.

Ex Output:
contracting


Ex Input:
Sentence1: if an object in a fluid is more dense than that fluid then that object will sink in that fluid because of gravity. 
Sentence2: Oil is less dense than water.

Ex Output:

dense