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: a fire requires oxygen to burn. 
Sentence2: Oxygen has  eight protons and eight electrons.

Output: oxygen


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: In Denver, traffic signal lights are being converted to the new energy-saving bulbs. 
Sentence2: Something in a traffic signal converts electrical energy into light energy when it is turned on.


Output: signal