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.

Example Input: Sentence1: Tree-ring dating the 1700 Cascadia earthquake. 
Sentence2: estimating the age of trees can be used in reviewing earthquake history.
Example Output: earthquake

Example Input: Sentence1: Eggs are usually laid in a nest. 
Sentence2: birds have nests.
Example Output: nest

Example Input: Sentence1: Opportunistic infections, such as Pneumocystis carinii pneumonia, begin to occur. 
Sentence2: Opportunistic infections may be a result of an infection or injury of the lungs.
Example Output:
Opportunistic