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: plant requires seed dispersal for reproduction. 
Sentence2: plants are pollinated by birds.
Example Output: plant

Example Input: Sentence1: harming an animal species has a negative impact on the population size of that species. 
Sentence2: the rarity of a species increases harm to population exponentially.
Example Output: harming

Example Input: Sentence1: a switch is used to start the flow of current in an electrical circuit. 
Sentence2: a switch is used to start the flow of current in a path of devices.
Example Output:
current