A Newton's cradle in the middle of a marble run. A marble runs down and strikes the first steel ball; the push goes through the row, the three in the middle stay perfectly still, and the last ball swings out and kicks the red marble waiting beyond it. The striker drops through a gap, rolls under the row and takes the red marble's place. A bucket chain carries every marble back to the top, and the bellhop rings his bell for each kick. Then the end ball's string starts to slip, the chain clanks and the lamp flickers. 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?