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: Smoking accelerates the aging process. 
Sentence2: smoking accelerates the death of cells.

SOLUTION: Smoking

PROBLEM: Sentence1: If there is sun, there is sunlight, there is day. 
Sentence2: Energy enters ecosystems from the sun.

SOLUTION: sun

PROBLEM: Sentence1: coal is used for generating heat by burning. 
Sentence2: When is coal burning CO 2 is given off.

SOLUTION:
burning