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: Lymphocytes are the key cells involved in the immune response. 
Sentence2: The key cells involved in the immune response produce antibodies.

Ex Output:
cells


Ex Input:
Sentence1: webbed feet are used for moving faster through water by aquatic animals. 
Sentence2: Feet are webbed and serve as a rudder.

Ex Output:

webbed