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: Amphibians are the first true tetrapods, or vertebrates with four limbs. 
Sentence2: Frogs are classified as amphibians.
Example Output: Amphibians

Example Input: Sentence1: a light bulb is used for seeing in the dark. 
Sentence2: light is required for seeing.
Example Output: seeing

Example Input: Sentence1: Fractures heal when osteoclasts form new bone. 
Sentence2: Fractures heal when osteoclasts form new living tissue.
Example Output:
Fractures