Conversation

The postal service illustrates good software interface design:

Senders only have to drop their message in any mailbox and can then walk away without a thought to whether the receiver moved or whether the message will be delivered by plane or by horseback.

Receivers only have to check their mailbox when they want to without a thought as to when the messages arrived, in what order, by what transportation method, by which delivery person, or from which point of origin.