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.

Ex Input:
Sentence1: An astronaut is a scientist who is a space explorer. 
Sentence2: Space explorers require preserved food for long flights.

Ex Output:
explorer


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

Ex Output:
ova


Ex Input:
Sentence1: a light bulb is used for seeing in the dark. 
Sentence2: Conventional light bulbs generate light by heating a filament.

Ex Output:

bulb