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: Nitrogen is found in proteins, nucleic acids, and chlorophyll. 
Sentence2: Nitrogen, as a gas, is colorless, odorless, and a generally inert element.
Example Output: Nitrogen

Example Input: Sentence1: propulsion is used for flying by a spacecraft. 
Sentence2: When Apollo 12 took off, the spacecraft was hit by lightning.
Example Output: spacecraft

Example Input: Sentence1: Each archegonium contains a single egg that is embedded in the prothallus. 
Sentence2: fertilization occurs when sperm swim inside a single egg embedded in the prothallus.
Example Output:
egg