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: friction causes an object to lose energy. 
Sentence2: Friction occurs when surfaces rub together.

Ex Output:
friction


Ex Input:
Sentence1: a flashlight converts chemical energy into light energy. 
Sentence2: PALight Flashlights are the ultimate personal LED flashlights.

Ex Output:
flashlight


Ex Input:
Sentence1: Arthropods have a life cycle with sexual reproduction. 
Sentence2: Insects have a life cycle with sexual reproduction.

Ex Output:

cycle