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: Telephone Telephone calls are expensive. 
Sentence2: Communicating by transmitting sound is expensive.

A: expensive
****
Q: Sentence1: a polar bear requires a cold environment. 
Sentence2: polar bears live where there are fewer animals.

A: bear
****
Q: Sentence1: hydropower requires damming a river. 
Sentence2: damming a river is a source of energy.

A:
damming
****