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: dew is formed when water vapor condenses over night. 
Sentence2: Condensing is a gas turning into a liquid.

Ex Output:
condenses


Ex Input:
Sentence1: Amphibians have permeable skin that easily absorbs substances from the environment. 
Sentence2: Some substances are harmful to the body and block biological systems.

Ex Output:
substances


Ex Input:
Sentence1: Bottleneck effect occurs when a population suddenly gets much smaller. 
Sentence2: Severe droughts have caused much of the rural population to urbanize.

Ex Output:

population