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: Plants use unsaturated fatty acids to store energy. 
Sentence2: Plants and fish contain fatty acids.
Example Output: Plants

Example Input: Sentence1: a thermometer is used to measure temperature. 
Sentence2: a thermometer is used to measure heat.
Example Output: a

Example Input: Sentence1: Most hormones are regulated by feedback mechanisms. 
Sentence2: Hormones are incredibly powerful chemicals.
Example Output:
hormones