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: Any antigen that causes an allergy is called an allergen. 
Sentence2: molds are antigens.
Example Output: antigen

Example Input: Sentence1: Clouds are one form of condensation. 
Sentence2: Clouds are one form of changing from a gas into a liquid by reducing heat energy.
Example Output: Clouds

Example Input: Sentence1: a solar panel converts sunlight into electricity. 
Sentence2: Lasers are powered by electricity.
Example Output:
electricity