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: Many birds perform elaborate courtship dances. 
Sentence2: Many birds perform dances before they mate.

SOLUTION: Many

PROBLEM: Sentence1: Combustion generates toxic fumes. 
Sentence2: High temperatures can cause toxic fumes.

SOLUTION: fumes

PROBLEM: Sentence1: Most mollusks have shells. 
Sentence2: Coral reefs and shells are made of calcium carbonate.

SOLUTION:
shells