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: Fertilization occurs naturally in the fallopian tubes. 
Sentence2: fertilization is used for reproduction.
Fertilization

Sentence1: Lions live in social groups called prides. 
Sentence2: Chimpanzees live in social groups called communities.
groups

Sentence1: cooking food requires adding heat energy. 
Sentence2: Heat energy makes digestion easier.

energy