recursive Instantiate will change prefab itself

Hi!
I try to write a script to teach students to learn recursion, and find this odd thing:

if I instantiate a gameobject itself and add the new object to its child, then the next time when I instentiate again, the gameobject will be cloned with all the new added children. Even I made a prefab and clone the prefab, it seems that it will modify the prefab. I want to create a fresh copy recursively. What shall I do? Thanks for the help!

hsin-chien
www.storynest.com