Share mutable data across threads: `Arc` for the shared ownership, `Mutex` for the synchronized access. Lock with `.lock().unwrap()`; the guard drops the lock on scope exit.
`Rc<T>` lets multiple owners share read-only access in single-threaded code; `Arc<T>` is the thread-safe version. Pair with `RefCell` / `Mutex` when you need shared MUTABLE access.