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: when electricity flows to a light bulb , the light bulb will turn on. 
Sentence2: water pumps can turn on light bulbs.

Ex Output:
bulb


Ex Input:
Sentence1: if an object is made of a material then that object has the properties of that material. 
Sentence2: An object is an item in the real world.

Ex Output:
object


Ex Input:
Sentence1: Population density is the average number of individuals per unit of area or volume. 
Sentence2: Economy is affected by population size and density.

Ex Output:

Population