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.

Example Input: Sentence1: Refraction is the bending of light. 
Sentence2: a rainbow is formed by bending of light by separating light into all different colors.
Example Output: bending

Example Input: Sentence1: a flashlight converts chemical energy into light energy. 
Sentence2: Only as long as there are batteries in the flashlight, the flashlight will function.
Example Output: flashlight

Example Input: Sentence1: Marble is a metamorphic rock. 
Sentence2: extreme heat and pressure change rock into marble.
Example Output:
Marble