The book of many sins

Knife

Cartman- If someone stabs you, then you own the knife as it has your DNA on it.
Clyde- That's how a collection starts!
Bebe- If you get stabbed and don't bleed out in 15 minutes you can legally leave.
Heidi*- If someone throws a knife and you catch it, they're out!
Kyle- By that logic, you have many places to catch knives with!

* Heidi is never fat Heidi unless specified