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.

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

Sentence1: harming an animal species causes that animal 's population to decrease. 
Sentence2: Oil spill sare harmful to animals and the enviroment.
harming

Sentence1: Animals can communicate with sounds, chemicals, or visual cues. 
Sentence2: Communication is the exchange of information, ideas and emotions.

communicate