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: burning fossil fuels increases the carbon in the atmosphere. 
Sentence2: the byproduct of fossil fuels is carbon.

Ex Output:
carbon


Ex Input:
Sentence1: microbial fuel cells convert chemical energy into electrical energy. 
Sentence2: microbial fuel cells convert chemical energy into kinetic energy.

Ex Output:
cells


Ex Input:
Sentence1: biofuel is used to produce electricity by burning. 
Sentence2: Wood and straw are both examples of biofuels.

Ex Output:

biofuel