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: mountains are formed by earthquakes. 
Sentence2: Mountains are formed by moving plates.
Example Output: are

Example Input: Sentence1: hunting requires seeing prey. 
Sentence2: hunting requires seeing an animal.
Example Output: hunting

Example Input: Sentence1: an organism requires energy for growth. 
Sentence2: an organism requires energy for something controlled by cones.
Example Output:
energy