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

Example Input: Sentence1: a flashlight requires a source of electricity to produce light. 
Sentence2: Flashlights provide the safest emergency lighting source.
Example Output: flashlight

Example Input: Sentence1: transpiration is when water vapor moves from plants into the atmosphere. 
Sentence2: transpiration is when H2O vapor moves from plants into the atmosphere.
Example Output:
atmosphere