Best way to puroposly crack the back of my iphone?

So I accidently dropped the front side of my iphone in class and it cracked. And I always wanted to try the "sharpie in cracks" kinda thing. But I don't wanna do it in the front considering the screen obviously. And since my front is already ruined I might aswell just crack the back and make it pretty. What's the best way to crack the back of my phone without damaging that inside of my phone? I want the cracks to be "spread out" so I don't want to hit it with a nail or something. And in the end if I don't like it I could always just replace the back or cover it with a case so

Added (1). I do have the Apple extended warrenty and my iphone backed up on my computer so if anything does go wrong I could just get a new phone at the Apple store for $50(: So I'm not THAT worried if it does die, I just rather it not lol

Added (2). Why? I just explained my whole reason in the details…

This sounds stupid but you could try that crackle nail polish haha, it might work… But I'm afraid that if you do try to actually crack it, you might mess up the inside of it and then it will just be useless.

Uh, why?