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: being burried under soil and mud changes peat into coal through extreme heat and pressure in a swamp over a long period of time. 
Sentence2: Marble is used for sculptures, and coal is used for fuel and energy like light.

Ex Output:
coal


Ex Input:
Sentence1: Bird eggs have hard, calcium carbonate shells. 
Sentence2: The reproductive cells of a bird is a hard, calcium carbonate shell.

Ex Output:
calcium


Ex Input:
Sentence1: Vertebrates are all chordates that have a backbone. 
Sentence2: birds have a backbone.

Ex Output:

backbone