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: Mutations are essential for evolution to occur. 
Sentence2: Genes are altered for evolution to occur.

Output: evolution


Input: Consider Input: Sentence1: when electricity flows to a light bulb , the light bulb will turn on. 
Sentence2: water pumps can turn on light bulbs.

Output: bulb


Input: Consider Input: Sentence1: Each starfish arm is lined with tube feet. 
Sentence2: Starfish are an echinoderm with a unique water vascular system.


Output: starfish