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: Enveloped viruses make their own membranes. 
Sentence2: Some viruses make their own membranes.

SOLUTION: make

PROBLEM: Sentence1: All life relies on a relatively narrow range of pH, or acidity. 
Sentence2: humans rely on a relatively narrow range of acidity.

SOLUTION: acidity

PROBLEM: Sentence1: Humans are also placental mammals. 
Sentence2: Humans are the dominant land mammals.

SOLUTION:
Humans