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: Female monotremes lack a uterus and vagina. 
Sentence2: Babies are normally delivered from the uterus through the vagina.

Ex Output:
uterus


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: Darwinism is strictly, selection theory. 
Sentence2: behaviors that improve fitness increase through Darwinism.

Ex Output:

Darwinism