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: Gravity is a force that pulls on things. 
Sentence2: A force that pulls on things pulls objects towards planets.

Ex Output:
force


Ex Input:
Sentence1: cold fronts cause thunderstorms as they pass by. 
Sentence2: Cold fronts cause lightning.

Ex Output:
cold


Ex Input:
Sentence1: DNA DNA DNA is a person's genetic fingerprint. 
Sentence2: Molecules of a person's genetic fingerprint are encoded with instructions that tell cells what to do.

Ex Output:

fingerprint