TASK DEFINITION: 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.
PROBLEM: Sentence1: Tree-ring dating the 1700 Cascadia earthquake. 
Sentence2: estimating the age of trees can be used in reviewing earthquake history.

SOLUTION: earthquake

PROBLEM: Sentence1: Herbs are medicines and aids. 
Sentence2: herbs are used for curing people when they are sick.

SOLUTION: Herbs

PROBLEM: Sentence1: Natural gas is a fossil fuel. 
Sentence2: heat and pressure changes fossils into fossil fuel.

SOLUTION:
fossil