This is a marble machine that you can build and rebuild over and over again on your fridge or any other magnetic surface.
I'm using neodymium magnets 10mm in diameter and 2-2.5 mm thick.
The blue tubing in the picture are left over protective tubing for from a water installation held up with hardrive magnets.
The parts are designed for marbles of about 16-17 mm in diameter.
I cut and glued an acrylic front on the marble divider, if you don't the marbles will fly everywhere :)