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: microscope is used to see small things by biologists by making them appear bigger. 
Sentence2: Something invented by Leeuwenhoek is used to see small things by biologists by making them appear bigger.
Example Output: appear

Example Input: Sentence1: an image in a mirror is formed by reflecting light. 
Sentence2: mirrors can be seen with our eyes.
Example Output: mirror

Example Input: Sentence1: Cells are the basic units of the structure and function of living things. 
Sentence2: life requires cells.
Example Output:
Cells