Definition: 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: 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.

caves