Use the arm to change levels, or link them together to form a chain reaction.
3D printed parts
Laser cut plywood
Neodymium magnets, and laser cut silicon non-slip stickers (so it grips the vertical surface)
Marble (for counter balance in base)
Instructional build video.
Build time: 20 mins
Paint brush and wood glue not included, but if you need them check my shop.