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: plant requires seed dispersal for reproduction. 
Sentence2: plants are pollinated by birds.

Ex Output:
plant


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

Ex Output:
thunderstorms


Ex Input:
Sentence1: Migration is a type of movement that changes population size. 
Sentence2: Birds migrate to escape poor weather and food supply.

Ex Output:

Migration