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: when a hemisphere is tilted away from the sun , that hemisphere receives less direct sunlight. 
Sentence2: Sunlight is delivered in small packets of energy called photons.
Example Output: sunlight

Example Input: Sentence1: rocks interacting with wind over long periods of time causes weathering. 
Sentence2: When a rock undergoes erosion and weathering, it breaks down to form sediments.
Example Output: weathering

Example Input: Sentence1: gravity causes objects that have mass to be pulled down on a planet. 
Sentence2: A force defined by Newton causes objects that have mass to be pulled down on a planet.
Example Output:
causes