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.

Ex Input:
Sentence1: riding a bike does not cause pollution. 
Sentence2: riding a bike does not release particles.

Ex Output:
bike


Ex Input:
Sentence1: harming an animal species has a negative impact on the population size of that species. 
Sentence2: Pesticides have a negative impact on the population size of a species.

Ex Output:
impact


Ex Input:
Sentence1: water vapor condensing in clouds causes rain. 
Sentence2: Water vapor is a gas in the air.

Ex Output:

vapor