Explain like I'm five: what is an API?

Recently I was reading this article about a big court case between google and oracle.

In it, they were trying to explain the concept of an API to the court. Some of their analogies did not go over very well!

From the article:

Eric Schmidt sought to describe APIs and languages using power plugs as an analogy. Jonathan Schwartz tried his hand at explaining with “breakfast menus,” only to have Judge William Alsup respond witheringly, “I don't know what the witness just said. The thing about the breakfast menu makes no sense.”

Schwartz’s second attempt at the breakfast menu analogy went much better, as he explained that although two different restaurants could have hamburgers on the menu, the actual hamburgers themselves were different—the terms on the menu were an API, and the hamburgers were implementations.

No one bothered to challenge Schwartz’s apparent belief that hamburgers are commonly featured on breakfast menus, as he had already moved on to confusing the jury on another front: the operating system GNU, which is a pearl of the free software community.

So basically, how would you explain what an API is better than Eric Schmidt?

I do basically understand what an API is, but I don't think I could explain the concept to anyone else, and I'm sure there are plenty of nerds around these parts, so I thought it might be a fun if someone else wants to take a crack at it.