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

Example Input: Sentence1: Diurnal cycles Diurnal cycles are the daily cycles of light and darkness. 
Sentence2: Plants detect and respond to diurnal cycles.
Example Output: Diurnal

Example Input: Sentence1: Fingernails prevent injury by forming protective plates over the ends of the fingers. 
Sentence2: For example, the finger is at the distal end of the human arm.
Example Output:
fingers