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: Metabolic rate is the rate at which food is burned for energy. 
Sentence2: thyroid hormones increase the rate of energy consumption.
Example Output: energy

Example Input: Sentence1: Examples of gametes are ova and spermatozoa. 
Sentence2: fertilization of ova and spermatozoa produces a diploid zygote.
Example Output: ova

Example Input: Sentence1: the surface of a beach is formed by mechanical weathering. 
Sentence2: Sand is formed by mechanical weathering.
Example Output:
formed