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: Sound vibrations can be compared to pond ripples. 
Sentence2: dropping an object into water causes pond ripples.

Ex Output:
pond


Ex Input:
Sentence1: cold fronts cause thunderstorms as they pass by. 
Sentence2: Thunderstorms create havoc.

Ex Output:
thunderstorms


Ex Input:
Sentence1: Evolution occurs by a process called natural selection. 
Sentence2: Evolution occurs by a sorting mechanism process.

Ex Output:

Evolution