Teacher: 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.
Teacher: Now, understand the problem? If you are still confused, see the following example:
Sentence1: pesticides cause pollution. 
Sentence2: pollution can harm animals.
Solution: pollution.
Reason: The word "pollution" is common to Sentence1 and Sentence2. So, it's a good answer.

Now, solve this instance: Sentence1: Tree-ring dating the 1700 Cascadia earthquake. 
Sentence2: estimating the age of trees can be used in reviewing earthquake history.
Student:
earthquake