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: the mass of a planet causes the pull of gravity on that planet. 
Sentence2: the mass of a planet is what causes things to have weight.
Example Output: mass

Example Input: Sentence1: Heat production is increased by shivering and non-shivering thermogenesis . 
Sentence2: Heat production occurs when many muscles contract a little bit all at once.
Example Output: Heat

Example Input: Sentence1: Cells are the microscopic building blocks of organisms. 
Sentence2: Cells are the microscopic building blocks of fish.
Example Output:
Cells