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: Urine leaves the body through a sphincter in the process of urination. 
Sentence2: urine leaves the body upon relaxation of a muscle.
Example Output: Urine

Example Input: Sentence1: high pressure systems cause clear weather. 
Sentence2: Weather Clear weather usually means good visibility.
Example Output: clear

Example Input: Sentence1: a cavern is formed by carbonic acid in groundwater seeping through rock and dissolving limestone. 
Sentence2: A cavern is formed when calcite is dissolved.
Example Output:
a