Couple
Tie the two; change one; let the other move only through the tie.
Two clocks on one beam fall into step.
Try it on your work
Hang two objects from the ends of a stick balanced on a string. Change one of them (add a clip, cut a piece away) and let the other move as it will. Stop after five changes.
Two people write side by side; every new sentence of each must take one word from the other's last sentence. Stop at ten sentences each.
Two people hold the ends of one rope and walk through a space. Neither may let go, and neither may speak. Stop after five minutes.
How it has been used
Tie two things so that a change in one moves the other, with nothing deciding in between: the two pendulums on one beam, the arms of a mobile, two pens tied with string. The move forbids steering the second thing directly: it moves only through the tie.
Where it leads
Grouped under Put different things into contact