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.

Input: Consider Input: Sentence1: a fire requires oxygen to burn. 
Sentence2: A fire requires something with exactly eight protons to burn.

Output: burn


Input: Consider Input: Sentence1: Cells are the microscopic building blocks of organisms. 
Sentence2: proteins are the building blocks of organisms.

Output: organisms


Input: Consider Input: Sentence1: Metazoan phylogeny Phylogeny of phyla is poorly understood. 
Sentence2: The evolutionary history of metazoas is poorly understood.


Output: Metazoan