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: Sperm pass out of the body through the urethra. 
Sentence2: Sperm travel from the testicles through the vas deferens to the seminal vesicles and the prostate.

Ex Output:
Sperm


Ex Input:
Sentence1: Mutations are essential for evolution to occur. 
Sentence2: Genes are altered for evolution to occur.

Ex Output:
evolution


Ex Input:
Sentence1: Cell death occurs by activation of cell death genes. 
Sentence2: Aging is associated with the activation of certain genes.

Ex Output:

activation