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: Antibiotics and vaccines are being stockpiled as Britain gears up to meet the bioterrorism threat. 
Sentence2: Antibiotics and vaccines are being stockpiled against another potential threat in the environment.

Ex Output:
Antibiotics


Ex Input:
Sentence1: Any antigen that causes an allergy is called an allergen. 
Sentence2: Allergens can trigger asthma attacks.

Ex Output:
allergen


Ex Input:
Sentence1: an animal requires warmth for survival. 
Sentence2: Vegetation is required by many animals for warmth and security.

Ex Output:

warmth