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 Q]: Sentence1: cold fronts cause thunderstorms as they pass by. 
Sentence2: Cold fronts cause lightning.
[EX A]: cold

[EX Q]: Sentence1: gills are used for breathing water by aquatic animals. 
Sentence2: gills are used for breathing water by sharks.
[EX A]: are

[EX Q]: Sentence1: Cancer is an example of a mutation in cells. 
Sentence2: Mutation can cause growth out of control.
[EX A]:
mutation