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: Caverns Secrets are hidden in caves. 
Sentence2: hidden caves are formed by carbonic acid in groundwater seeping through rock and dissolving limestone.

Ex Output:
are


Ex Input:
Sentence1: harming an animal species has a negative impact on the population size of that species. 
Sentence2: the rarity of a species increases harm to population exponentially.

Ex Output:
harming


Ex Input:
Sentence1: Bird eggs have hard, calcium carbonate shells. 
Sentence2: Birds lay clutches with hard, calcium carbonate shells.

Ex Output:

calcium