You are presented with the quiz "What breaks up into two lighter elements when an animal divides into two parts?" 

But you don't know the answer, so you turn to your teacher to ask for hints. He says that "fission takes place when an animal divides into two parts" and "if the nucleus breaks up into lighter elements, it is called fission ". 

So, what's the best answer to the question?


 

       the nucleus    