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: Organisms cluster around hydrothermal vents in the ocean floor. 
Sentence2: vents on the ocean floor are powered by heat.
Example Output: vents

Example Input: Sentence1: Rainforests are destroyed by logging and all of the trees are cut down. 
Sentence2: Logging  down trees destroys rainforests.
Example Output:
Rainforests