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: boiling is when liquids are heated above their boiling point. 
Sentence2: If the pressure is reduced, the boiling point is lowered.
Example Output: boiling

Example Input: Sentence1: Fish have a centralized nervous system with a brain. 
Sentence2: fish are vertebrates.
Example Output:
Fish