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 light bulb is used for seeing in the dark. 
Sentence2: light is required for seeing.

Output: seeing


Input: Consider Input: Sentence1: Speciation events are thus examples of macroevolution. 
Sentence2: Speciation events occur over geologic time above the level of the species.

Output: Speciation


Input: Consider Input: Sentence1: if gas is heated then that gas will expand. 
Sentence2: When a gas expands, it cools off.


Output: expand