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

Sentence1: Trees have woody stems covered with bark. 
Sentence2: Evergreens have woody stems covered with bark.
bark

Sentence1: Life in a volcanic environment can nevertheless be treacherous for animals. 
Sentence2: There is little wildlife in volcanic areas.

volcanic