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.

[Q]: Sentence1: Coral reefs provide food and shelter to many ocean organisms. 
Sentence2: Coral Reefs Coral reefs are threatened.
[A]: Coral


[Q]: Sentence1: Oxygen fatigue is a shortage of oxygen reaching the brain. 
Sentence2: Plants can reduce fatigue during the daytime.
[A]: fatigue


[Q]: Sentence1: Most ecosystems get energy from sunlight. 
Sentence2: Plants are part of the ecosystem.
[A]:
ecosystems