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: a mutation in the sex cells of a parent can cause a new trait to appear in the parent 's offspring. 
Sentence2: cancer of a parent's cells can cause a new trait to appear in their offspring.

SOLUTION: appear

PROBLEM: Sentence1: Movement is thought in motion. 
Sentence2: migration is a type of motion that changes population size.

SOLUTION:
motion