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: Apollo 13 spacecraft is launched. 
Sentence2: Propulsion was used for flying Apollo 13.
Example Output: 13

Example Input: Sentence1: Warm temperatures have helped plant growth and sizing. 
Sentence2: a greenhouse helps  plant growth.
Example Output: growth

Example Input: Sentence1: Viruses are usually considered to be nonliving. 
Sentence2: acellular organisms need a living host.
Example Output:
nonliving