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: a greenhouse is used to protect plants by keeping them warm. 
Sentence2: Flower size varies by plant.

SOLUTION: plants

PROBLEM: Sentence1: Secondary accidents are a major problem under driving conditions of reduced visibility. 
Sentence2: Bad weather increases the risk of car accidents.

SOLUTION: Secondary

PROBLEM: Sentence1: Carbon may be stored in sedimentary rock for millions of years. 
Sentence2: carbon may be stored in limestone for million of years.

SOLUTION:
Carbon