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: Cells are the units of function in living things. 
Sentence2: living things cannot survive without oxygen.

Ex Output:
living


Ex Input:
Sentence1: Any antigen that causes an allergy is called an allergen. 
Sentence2: Antigens are proteins foreign to the body.

Ex Output:
antigen


Ex Input:
Sentence1: Nitrogen gas is the most abundant gas in the atmosphere. 
Sentence2: if nitrogen is heated it will expand.

Ex Output:

Nitrogen