TASK DEFINITION: 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.
PROBLEM: Sentence1: All chemical reactions require activation energy to get started. 
Sentence2: Catalysts lower the activation energy.

SOLUTION: activation

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

SOLUTION: cold

PROBLEM: Sentence1: a warm front causes cloudy and rainy weather. 
Sentence2: Cirrus clouds can indicate the arrival of rainy weather.

SOLUTION:
rainy