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.

Example Input: Sentence1: Interstellar space contains a plasma. 
Sentence2: interstellar space contains electrons, atoms, and stars.
Example Output: Interstellar

Example Input: Sentence1: dropping an object into water causes vibrations in the water. 
Sentence2: Dropping an object into lake causes vibrations.
Example Output: dropping

Example Input: Sentence1: a fire requires oxygen to burn. 
Sentence2: a fire requires a by-product of photosynthesis.
Example Output:
a