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: Viruses contain DNA but not much else. 
Sentence2: viruses contain encoded information but not much else.

SOLUTION: Viruses

PROBLEM: Sentence1: Human mating behavior also involves some competition. 
Sentence2: Human mating may lead to aggressive behaviors or displays of aggresion.

SOLUTION: Human

PROBLEM: Sentence1: Some analytical tools, such as optical microscopes, are easy to imagine. 
Sentence2: Microscopes enable a person to observe a material up close.

SOLUTION:
microscopes