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.

Input: Consider Input: Sentence1: An astronaut is a scientist who is a space explorer. 
Sentence2: Space explorers require preserved food for long flights.

Output: explorer


Input: Consider Input: Sentence1: Sound vibrations can be compared to pond ripples. 
Sentence2: dropping an object into water causes pond ripples.

Output: pond


Input: Consider Input: Sentence1: When caves contain connecting rooms, they are called caverns. 
Sentence2: Caves can be formed by carbonic acid in groundwater seeping through rock and dissolving limestone.


Output: caves