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: Mitosis is cell division. 
Sentence2: mitosis often causes growth.
Example Output: Mitosis

Example Input: Sentence1: freezing means changing from a liquid into a solid by reducing heat energy. 
Sentence2: Water freezes into ice when cooled.
Example Output: freezing

Example Input: Sentence1: Oxygen enters the respiratory system through the mouth and the nose. 
Sentence2: Breathing masks are used to keep harmful particles from entering the mouth and the nose.
Example Output:
mouth