It might be nice if you could tell by looking at the toy that it was moving, and not have to wait for it to tell you so. There are a number of relevant differences between this and most other verbs, however. Most verbs do something, that is, when you type a command, it produces a set of messages, and changes some properties on some objects. Some verbs, however, are not commands, and don't even print any messages. Instead, they return a value to the calling verb, for that verb to use to produce messages or change properties.
Description is one such verb.
By default, that is, in the basic case, it returns the property this.description . Verbs like look call description on objects and print out the strings they return. Again because we're trying to write generically, we create a property to hold a message to be added to the description when the toy is running, and we set it for the duck. First we call pass to get the underlying description. We store away that text in a variable basic , because we'll need it later.