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.

Sentence1: Parasites are organisms that live off a host. 
Sentence2: roundworms may be free-living or live off a host.
host

Sentence1: Sperm had to swim from male to female reproductive organs for fertilization. 
Sentence2: If fertilized, the egg implants itself in the uterus.
fertilization

Sentence1: if a neutral atom loses an electron then an atom with a negative charge will be formed. 
Sentence2: An ion is formed after a neutral atom loses an electron.

atom