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.

Ex Input:
Sentence1: Carpools and vanpools can save time. 
Sentence2: vanpools decrease the amount of cars used to travel to a place.

Ex Output:
vanpools


Ex Input:
Sentence1: a mutation in the sex cells of a parent can cause a new trait to appear in the parent 's offspring. 
Sentence2: Traits are initiated by mutation and recombination.

Ex Output:
mutation


Ex Input:
Sentence1: a cavern is formed by carbonic acid in groundwater seeping through rock and dissolving limestone. 
Sentence2: Carbon dioxide dissolved in rainwater forms carbonic acid.

Ex Output:

acid