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.

Q: Sentence1: cold fronts cause thunderstorms as they pass by. 
Sentence2: Cold fronts cause lightning.

A: cold
****
Q: Sentence1: friction causes the temperature of an object to increase. 
Sentence2: Friction causes the temperature of a toy to increase.

A: causes
****
Q: Sentence1: Oncogenes promote the division of cells with damaged DNA. 
Sentence2: DNA DNA DNA is a person's genetic fingerprint.

A:
DNA
****