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: harming an animal species has a negative impact on the population size of that species. 
Sentence2: the rarity of a species increases harm to population exponentially.
Example Output: harming

Example Input: Sentence1: Animals can communicate with sounds, chemicals, or visual cues. 
Sentence2: cats can communicate with sounds, chemicals or visual cues.
Example Output: chemicals

Example Input: Sentence1: Carbon may be stored in sedimentary rock for millions of years. 
Sentence2: Carbon may be stored in marble for years.
Example Output:
Carbon