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: burning fossil fuels increases the carbon in the atmosphere. 
Sentence2: the byproduct of fossil fuels is carbon.

SOLUTION: carbon

PROBLEM: Sentence1: Apollo 13 spacecraft is launched. 
Sentence2: Propulsion was used for flying Apollo 13.

SOLUTION: 13

PROBLEM: Sentence1: Viruses attach themselves to cells and reproduce. 
Sentence2: Viruses cannot make copies of themselves alone. They need to attach themselves to cells.

SOLUTION:
attach