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.

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

Output: activation


Input: Consider Input: Sentence1: a fire requires oxygen to burn. 
Sentence2: If the oxygen is removed, the fire is smothered.

Output: oxygen


Input: Consider Input: Sentence1: Adolescence is the last stage of life before adulthood. 
Sentence2: Life is very complex for the adolescent.


Output: Adolescence