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: Ferns are vascular plants. 
Sentence2: when a fern grows, its size of leaves may increase.

SOLUTION: Ferns

PROBLEM: Sentence1: friction is used for stopping a vehicle by brakes. 
Sentence2: friction is used for stopping a car by brakes.

SOLUTION: brakes

PROBLEM: Sentence1: telephones are used for communication by two people by transmitting sound. 
Sentence2: Generally, a telephone is any device which conveys sound over a distance.

SOLUTION:
sound