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: if an animal lives under ground then that animal will be protected from a wild fire. 
Sentence2: animals that live under ground in California will be protected from some natural disasters.
Example Output: animal

Example Input: Sentence1: Results can be damaging or disastrous. 
Sentence2: Lack of immunization can be damaging or disastrous.
Example Output: damaging

Example Input: Sentence1: Plants release oxygen during the day but not during the night. 
Sentence2: Oxygen fatigue is a shortage of oxygen reaching the brain.
Example Output:
oxygen