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: Human mating behavior also involves some competition. 
Sentence2: Human mating may lead to aggressive behaviors or displays of aggresion.
Example Output: Human

Example Input: Sentence1: natural magnetism is used for pointing north by a compass. 
Sentence2: For the ordinary compass, align to the North direction.
Example Output: north

Example Input: Sentence1: propulsion is used for flying by a spacecraft. 
Sentence2: All rockets operate on the same basic principles of propulsion.
Example Output:
propulsion