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: Pregnancy is the carrying of one or more offspring from fertilization until birth. 
Sentence2: An elephant carries their offspring from fertilization until birth, 22 months later.

Ex Output:
fertilization


Ex Input:
Sentence1: Humans have collected and grown mushrooms for food for thousands of years. 
Sentence2: Yeast and mushrooms are types of fungus.

Ex Output:
mushrooms


Ex Input:
Sentence1: Active immunity results when an immune response to a pathogen produces memory cells. 
Sentence2: Memory cells are the basis of immunological memory.

Ex Output:

cells