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 accounting period is simply a period of time. 
Sentence2: seconds are used to measure a period.
Example Output: period

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: when a hemisphere is tilted away from the sun , that hemisphere receives less direct sunlight. 
Sentence2: Similarly, stars are referred to as suns, but our star is the Sun.
Example Output:
sun