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: Instead, the body is covered with a protective shell of bony armor. 
Sentence2: Most mollusks have protective of bony armor.
Example Output: armor

Example Input: Sentence1: Sperm had to swim from male to female reproductive organs for fertilization. 
Sentence2: If fertilized, the egg implants itself in the uterus.
Example Output: fertilization

Example Input: Sentence1: Ethologists usually study how animals behave in their natural environment. 
Sentence2: Ethologists usually study how animals behave in their homes in an ecosystem.
Example Output:
Ethologists