Also you can steal currency. At this very moment, I owe $8 to my friend because she bought me a drink last night. It is not possible for anyone to steal that $8 because it's a purely relational construct: I and she both know that I owe her $8. I can default, thereby deprive my friend what she's owed, but no third party can swoop in and take the money away from her. But in contrast, you can steal 8 $1 bills quite easily.