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: Rivers, rainwater, well water and most lakes contain fresh water. 
Sentence2: canyons are formed by rainwater.

Ex Output:
rainwater


Ex Input:
Sentence1: If there is sun, there is sunlight, there is day. 
Sentence2: Energy enters ecosystems from the sun.

Ex Output:
sun


Ex Input:
Sentence1: harming an animal species has a negative impact on the population size of that species. 
Sentence2: harming a kind of animal has a negative impact on the population size of that animal.

Ex Output:

animal