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: dropping an object into water causes vibrations in the water. 
Sentence2: Dropping an object into lake causes vibrations.

SOLUTION: dropping

PROBLEM: Sentence1: threatening behavior by predators causes a porcupine to extend its quills. 
Sentence2: Fishers are predators that can eat porcupines.

SOLUTION: predators

PROBLEM: Sentence1: a compass is used for determining direction. 
Sentence2: a compass is used for navigation.

SOLUTION:
a