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: threatening behavior by predators causes a porcupine to extend its quills. 
Sentence2: Fishers are predators that can eat porcupines.

Ex Output:
predators


Ex Input:
Sentence1: propulsion is used for flying by a spacecraft. 
Sentence2: Jets are used for flying by spacecraft.

Ex Output:
flying


Ex Input:
Sentence1: DNA condenses to form chromosomes. 
Sentence2: Genectic material is compressed to form chromosomes.

Ex Output:

form