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: Habitat destruction is the primary cause of extinction or threat of extinction for all animals. 
Sentence2: building housing developments may cause extinction.

Answer: extinction


Question: Sentence1: Telephone Telephone calls are expensive. 
Sentence2: Communicating by transmitting sound is expensive.

Answer: expensive


Question: Sentence1: Cells are the basic units of the structure and function of living things. 
Sentence2: Cells are the basic units of the structure and are biodegradable.

Answer:
Cells