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: Heterotrophs are living things that cannot make their own food. 
Sentence2: food contains organic molecules.
Example Output: food

Example Input: Sentence1: Finally, the fluid in the nephron reaches a collecting duct . 
Sentence2: fluid in the kidneys collect into ducts.
Example Output: fluid

Example Input: Sentence1: air pollution can cause the pH of soil to decrease. 
Sentence2: Some soils are more sensitive than others to the acid dropped by air pollution.
Example Output:
air