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: Any antigen that causes an allergy is called an allergen. 
Sentence2: Antigens are proteins foreign to the body.
Example Output: antigen

Example Input: Sentence1: Streams may start with runoff or water seeping out of a spring. 
Sentence2: streams may start with runoff or h2o seeping out of a spring.
Example Output: Streams

Example Input: Sentence1: Alternation of generations allows for both asexual and sexual reproduction. 
Sentence2: Reproduction Organisms create young of their species by reproduction.
Example Output:
reproduction