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: hunting requires seeing prey. 
Sentence2: hunting requires seeing an animal.
Example Output: hunting

Example Input: Sentence1: ultraviolet light causes sunburn. 
Sentence2: Sunburn risk is higher at elevation.
Example Output: sunburn

Example Input: Sentence1: Birds evolved from a group of bipedal dinosaurs called theropods. 
Sentence2: Feathers are highly probable in the evolution of a group of bipedal dinosaurs.
Example Output:
bipedal