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: propulsion is used for flying by a spacecraft. 
Sentence2: Jets are used for flying by spacecraft.

Ex Output:
flying


Ex Input:
Sentence1: Ice is frozen water. 
Sentence2: ice expands.

Ex Output:
Ice


Ex Input:
Sentence1: Humans use evaporation of water, sweat, to cool their bodies. 
Sentence2: evaporation cools the body.

Ex Output:

evaporation