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: plant requires seed dispersal for reproduction. 
Sentence2: plants are pollinated by birds.
Example Output: plant

Example Input: Sentence1: Scientists still know relatively little about Archaea. 
Sentence2: scientists still know relatively little about non-bacterial prokaryotes.
Example Output: Scientists

Example Input: Sentence1: Ethologists usually study how animals behave in their natural environment. 
Sentence2: animal behaviorists study animals in their natural environment.
Example Output:
animals