instruction:
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.
question:
Sentence1: Telephone Telephone calls are expensive. 
Sentence2: Communicating by transmitting sound is expensive.
answer:
expensive


question:
Sentence1: friction causes an object to lose energy. 
Sentence2: Friction occurs when surfaces rub together.
answer:
friction


question:
Sentence1: threatening behavior by predators causes a porcupine to extend its quills. 
Sentence2: Porcupines are in the rodent family.
answer:

porcupine