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: Very often their only metal object in the house is a cooking pot or a knife. 
Sentence2: sharpening a knife causes it to change shape.
Example Output: knife

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: Male gametes are called sperm. 
Sentence2: Pollen consists of tiny grains that are the sperm. of plants.
Example Output:
sperm