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.

Example Input: Sentence1: an animal usually requires a warm body temperature for survival. 
Sentence2: Warm blooded animals have a constant body temperature.
Example Output: animal

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

Example Input: Sentence1: Small lymph capillaries collect lymph. 
Sentence2: small capillaries collect fluid.
Example Output:
Small