Pole to Pole

Three magnets sit on a rail beside one that is clamped down, and dashed marks show where each of them has to end up. You cannot push a magnet along the rail. The only thing you can do is turn one round, and then the magnets move themselves: a north end and a south end clasp together, while two ends that are the same shove each other apart.

The catch is that a magnet has two ends and they are opposites, so turning one changes the joint on BOTH sides of it. Closing one gap opens the next. The second board wants a gap at each end and the middle pair still holding, which means making like ends meet on purpose.

Then a tray of six things and a magnet bolted to a bench: an iron nail, a steel paperclip, some aluminium foil, a copper coin, a plastic brick and a second bar magnet. You say what the bench will do to each one before it is tested. Two of those metals are not pulled at all, and the second magnet is not a fixed answer either: it can be pulled in or pushed away, and which one is up to you.