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

A: earthquake
****
Q: Sentence1: erosion causes a river to become deeper and wider. 
Sentence2: On the contrary, the NILE is the longest river in Africa.

A: river
****
Q: Sentence1: Chemical digestion occurs mainly in the small intestine. 
Sentence2: Extracellular digestion begins in the pharynx, with further digestion in the stomodaeum.

A:
digestion
****