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: the mass of a planet causes the pull of gravity on that planet. 
Sentence2: the mass of a planet is what causes things to have weight.
Example Output: mass

Example Input: Sentence1: Insects spread disease and destroy crops. 
Sentence2: Crops are plants that are grown for food or other uses.
Example Output: crops

Example Input: Sentence1: the number of rings in a tree trunk can be used to estimate the age of the tree. 
Sentence2: Dendrochronologist: A person who studies tree ring growth.
Example Output:
rings