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: a fire requires oxygen to burn. 
Sentence2: Oxygen has  eight protons and eight electrons.
Example Output: oxygen

Example Input: Sentence1: rain causes erosion. 
Sentence2: Soil Erosion Soil erosion is what happens when soil is washed or blown away.
Example Output: erosion

Example Input: Sentence1: Mitosis is cell division. 
Sentence2: Eukaryotes have more complex mitosis than prokaryotes.
Example Output:
Mitosis