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: Ferns are vascular plants. 
Sentence2: when a fern grows, its size of leaves may increase.

SOLUTION: Ferns

PROBLEM: Sentence1: All sharks are predators. 
Sentence2: hiding can be used for avoiding sharks.

SOLUTION: sharks

PROBLEM: Sentence1: Flatworms Flatworms or Platyhelminthes include flukes, tapeworms, and turbellarians. 
Sentence2: Tapeworms belong to the phylum Platyhelminthes.

SOLUTION:
tapeworms