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.

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

A: fertilization
****
Q: Sentence1: Light source is an ultra-bright, unbreakable LED Bulb. 
Sentence2: LEDs convert electricity into light and heat energy.

A: LED
****
Q: Sentence1: Vertebrate chordates have a backbone. 
Sentence2: reptiles have a backbone.

A:
backbone
****