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: Most copepods feed on phytoplankton. 
Sentence2: most copepods feed on bacteria and algae that use sunlight to make food.

Ex Output:
copepods


Ex Input:
Sentence1: All chemical reactions require activation energy to get started. 
Sentence2: Catalysts lower the activation energy.

Ex Output:
activation


Ex Input:
Sentence1: Bioterrorism is another potential threat in the environment. 
Sentence2: Where there is an environment, there is life within it.

Ex Output:

environment