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: Very often their only metal object in the house is a cooking pot or a knife. 
Sentence2: sharpening a knife causes it to change shape.

Ex Output:
knife


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

Ex Output:
antigen


Ex Input:
Sentence1: Aquatic creatures live in the water, for example. 
Sentence2: Algae play significant roles as producers in water ecosystems.

Ex Output:

water