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: Mammals that are viviparous are called therian mammals. 
Sentence2: Mouse-sized viviparous mammals evolved by Jurassic times.

Ex Output:
Mammals


Ex Input:
Sentence1: a plant requires sunlight to grow. 
Sentence2: Grass needs air and sunlight.

Ex Output:
sunlight


Ex Input:
Sentence1: Mollusks are bilateraly symetrical and are protostomes. 
Sentence2: The common snail is bilaterally symmetrical and a protostome.

Ex Output:

bilateraly