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: creating wood-free paper does not require cutting down trees. 
Sentence2: creating printed media requires cutting down trees.

Ex Output:
creating


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: Fish have a centralized nervous system with a brain. 
Sentence2: Amphetamines stimulate the central nervous system.

Ex Output:

centralized