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.

Example Input: Sentence1: Introns are regions that do not code for proteins. 
Sentence2: regions that do not code for proteins are removed by RNA splicing.
Example Output: code

Example Input: Sentence1: Telephone Telephone calls are expensive. 
Sentence2: Communicating by transmitting sound is expensive.
Example Output: expensive

Example Input: Sentence1: a solar panel converts sunlight into electricity. 
Sentence2: Solar energy is converted directly to electricity by the photovoltaic array.
Example Output:
solar