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: cell division often causes growth. 
Sentence2: Mitosis is cell division.

A: cell
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Q: Sentence1: Tree-ring dating the 1700 Cascadia earthquake. 
Sentence2: estimating the age of trees can be used in reviewing earthquake history.

A: earthquake
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Q: Sentence1: Volume is the amount of space an object contains. 
Sentence2: the amount of space an object takes up is its size.

A:
amount
****