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: Herbs are medicines and aids. 
Sentence2: herbs are used for curing people when they are sick.
Example Output: Herbs

Example Input: Sentence1: Ions are atoms or groups of atoms that carry a positive or negative electrical charge. 
Sentence2: An ion is formed after a neutral atom loses an electron.
Example Output: Ions

Example Input: Sentence1: the number of rings in a tree trunk can be used to estimate the age of the tree. 
Sentence2: Tree rings record the seasons.
Example Output:
rings