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: Heat production is increased by shivering and non-shivering thermogenesis . 
Sentence2: Heat production occurs when many muscles contract a little bit all at once.

Ex Output:
Heat


Ex Input:
Sentence1: An example of a cartilaginous fish is a shark. 
Sentence2: jaws make sharks excellent predators.

Ex Output:
shark


Ex Input:
Sentence1: the looseness of soil increases the amount of oxygen in that soil. 
Sentence2: Oxygen, as you know, is a gas.

Ex Output:

oxygen