You use `Rc` when a value can have more than one owner. You use `RefCell` when you need multiple mutable references to a value in scope (aka interior mutability

You use `Rc` when a value can have more than one owner. You use `RefCell` when you need multiple mutable references to a value in scope (aka interior mutability - there are more than one mutable references held, but the borrow checker can't prove that they don't overlap at compile time).