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: a fire requires oxygen to burn. 
Sentence2: Oxygen has  eight protons and eight electrons.

Ex Output:
oxygen


Ex Input:
Sentence1: Mitosis is cell division. 
Sentence2: Chromosomes are the form of the genetic material of a cell during mitosis.

Ex Output:
Mitosis


Ex Input:
Sentence1: better senses have a positive impact on an animal 's survival. 
Sentence2: Dogs are social animals, but they have an acute sense of smell.

Ex Output:

animal