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: Very often their only metal object in the house is a cooking pot or a knife. 
Sentence2: sharpening a knife causes it to change shape.

Output: knife


Input: Consider Input: Sentence1: Sperm had to swim from male to female reproductive organs for fertilization. 
Sentence2: If fertilized, the egg implants itself in the uterus.

Output: fertilization


Input: Consider Input: Sentence1: Sound Sound waves are mechanical waves. 
Sentence2: matter vibrating can cause mechanical waves.


Output: mechanical