A marble run whose wire is a trefoil knot: one closed loop that passes over and under itself three times and never touches. A knot cannot be a run with only one lift, so two giant brass knitting needles swing the marbles back up, one at each end of the two runs, while a tin grandmother rocks and knits along with them. Follow the red marble: over, under, over. Then something starts to rattle. 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?