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.

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

Example Input: Sentence1: Ejaculation allows males to reproduce. 
Sentence2: propelling sperm from the epididymes allows males to reproduce.
Example Output: allows

Example Input: Sentence1: killing microorganisms in food can be used to extend the shelf life of that food. 
Sentence2: Irradiation kills microorganisms that spoil food.
Example Output:
killing