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: an animal usually requires a warm body temperature for survival. 
Sentence2: Warm blooded animals have a constant body temperature.
Example Output: animal

Example Input: Sentence1: Coral reefs are made up of the accumulated exoskeletons of the coral polyp. 
Sentence2: coral polyps form reefs.
Example Output: polyp

Example Input: Sentence1: Bird Migration and Isotopes Tracking animal migration is a difficult task. 
Sentence2: Earth 's magnetic patterns are used for finding locations by birds.
Example Output:
Bird