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: Mammals depend on the vegetation for food and shelter. 
Sentence2: Mammals are Heterotrophs.
Example Output: Mammals

Example Input: Sentence1: triceps contracting causes a person 's elbow to straighten. 
Sentence2: muscles contracting cause a person's elbow to straighten.
Example Output: contracting

Example Input: Sentence1: Fertilization As far as fertilization, timing is critical. 
Sentence2: pollination timing is a critical step in the sexual reproduction of plants.
Example Output:
critical