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.
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Question: Sentence1: If there is sun, there is sunlight, there is day. 
Sentence2: Energy enters ecosystems from the sun.

Answer: sun


Question: Sentence1: Eggs are usually laid in a nest. 
Sentence2: birds have nests.

Answer: nest


Question: Sentence1: Polluted rain is called acid rain . 
Sentence2: sulfur dioxide causes pollution.

Answer:
Polluted