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: All frogs are amphibians. 
Sentence2: frogs have permeable skin that easily absorbs substances from the environment.

Ex Output:
frogs


Ex Input:
Sentence1: Lancelet larvae are free-swimming. 
Sentence2: some larvae lack a hardened backbone.

Ex Output:
larvae


Ex Input:
Sentence1: Ancestors of hagfish are thought to have been the earliest vertebrates. 
Sentence2: All vertebrates, except for the hagfish and lampreys, have alpha and beta forms, .

Ex Output:

vertebrates