A drive wheel turns all the time. Its one pin slides into a slot of the six-slot star, turns it a sixth and slides out, and the drive's round lock holds the star still until the pin comes round again. So the big wheel steps and stops, steps and stops. At every stop one marble rolls into a notch at the bottom and one is lifted out at the top, down the left side and back into the queue. Seven marbles, no job. The birds on the cord have seen it all before, until the drive's pin starts to creak. Stay for the end.

Every knock and every music-box note is timed by the machine itself.

Which machine with no job should we build next?