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: pathogens can cause disease. 
Sentence2: pathogens affects the immune system.

SOLUTION: pathogens

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: a beak is used for catching prey by some birds. 
Sentence2: Bird beaks are multi-functional tools.

SOLUTION:
birds