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.

Ex Input:
Sentence1: Ferns are vascular plants. 
Sentence2: when a fern grows, its size of leaves may increase.

Ex Output:
Ferns


Ex Input:
Sentence1: a mutation in the sex cells of a parent can cause a new trait to appear in the parent 's offspring. 
Sentence2: Traits are initiated by mutation and recombination.

Ex Output:
mutation


Ex Input:
Sentence1: DNA is an abbreviation for deoxyribonucleic acid. 
Sentence2: Chromosomes are coiled structures made of deoxyribonucleic acid and proteins.

Ex Output:

acid