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: a light bulb is used for seeing in the dark. 
Sentence2: light is required for seeing.

Ex Output:
seeing


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: Carbon cycles quickly between organisms and the atmosphere. 
Sentence2: An organic molecule is one involving carbon.

Ex Output:

Carbon