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.

Input: Consider Input: Sentence1: Any antigen that causes an allergy is called an allergen. 
Sentence2: molds are antigens.

Output: antigen


Input: Consider Input: Sentence1: Cells are the units of function in living things. 
Sentence2: living things cannot survive without oxygen.

Output: living


Input: Consider Input: Sentence1: All frogs are amphibians. 
Sentence2: Frogs breathe with gills as larvae and with lungs as adults.


Output: frogs