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: Bird eggs have hard, calcium carbonate shells. 
Sentence2: The reproductive cells of a bird is a hard, calcium carbonate shell.

SOLUTION: calcium

PROBLEM: Sentence1: Skin color is another polygenic trait. 
Sentence2: Skin color is influenced by multiple genes.

SOLUTION: Skin

PROBLEM: Sentence1: For a male, urine markings are used to define the boundaries of their territory. 
Sentence2: male dogs use pheromones in urine markings to define boundaries.

SOLUTION:
boundaries