I have often enjoyed the use of analogies in understanding a software scenario or problem.

For example, to understand the concept of public key encryption, the 'locked mailbox' analogy or similar is often used as an aid:

An analogy for public-key encryption is that of a locked mailbox with a mail slot. The mail slot is exposed and accessible to the public; its location (the street address) is in essence the public key. Anyone knowing the street address can go to the door and drop a written message through the slot; however, only the person who possesses the key can open the mailbox and read the message.

My question is:

What analogies have you used or heard of in your career that have given you that "Eureka" moment with a complex concept?

EDIT: If you have a good one, don't just state the name, please share with the group!