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: Some pelycosaurs gave rise to a group of animals called therapsids. 
Sentence2: Like most other groups of synapsids, all pelycosaurs are now extinct.

Ex Output:
pelycosaurs


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: burning wood is used to produce heat. 
Sentence2: Heating and air conditioning keep the building comfortable regardless of the temperature outside.

Ex Output:

heat