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: drought is when amount of precipitation decreases. 
Sentence2: Drought decreases the availability and abundance of the natural food supply.

Output: drought


Input: Consider Input: Sentence1: Bird eggs have hard, calcium carbonate shells. 
Sentence2: The reproductive cells of a bird is a hard, calcium carbonate shell.

Output: calcium


Input: Consider Input: Sentence1: Mammals can also generate little bursts of heat by shivering. 
Sentence2: Heat is produced by muscle action and shivering.


Output: shivering