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: a rainbow is formed by refraction of light by separating light into all different colors. 
Sentence2: Rainbows form when raindrops refract sunlight.

Answer: refraction


Question: Sentence1: An accounting period is simply a period of time. 
Sentence2: seconds are used to measure a period.

Answer: period


Question: Sentence1: Polyps usually reproduce asexually. 
Sentence2: Reefs are built by tiny organisms called polyps.

Answer:
Polyps