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: Leeches are external parasites. 
Sentence2: Leeches live on the surface of their hosts.

SOLUTION: Leeches

PROBLEM: Sentence1: Among monogamous species of birds and mammals, both parents often care for their offspring. 
Sentence2: The period of parental care after birth for manogamous species are handled by both parents.

SOLUTION: monogamous

PROBLEM: Sentence1: Porcupine quills can be extraordinarily painful. 
Sentence2: threatening behavior can be painful.

SOLUTION:
painful

