Physical analogue for MACs What would be a good analogue with which to describe Message Authentication Codes to a person who has little to no understanding of cryptography?

For instance, a vault is a reasonable analogue for symmetric authenticated encryption: anyone with the key can open the vault and inspect, alter, or replace the contents. Without the key, you can't know or manipulate what's inside. Similarly, a royal wax seal is an okay one for digital signatures: anyone can validate that a message originated from the person with that particular stamp with assurances that the message hasn't been altered since the envelope was sealed.

Is there a good physical example of something with similar semantics to MACs?
Just thinking out loud here:


Take a picture of the contents of a box.
Put these pictures in a safe.
Ship box and safe together, lock with key of sender.
Receiver unlocks safe, compares pictures with contents of box.


The safe and keys are already common for symmetric encryption too.