Things you will need:
A metal marble 10mm:https://www.amazon.com/10mm-Chrome-steel-bearing-balls/dp/B008M35ESA/ref=sr_1_fkmr1_3?s=industrial&ie=UTF8&qid=1516562962&sr=1-3-fkmr1&keywords=10mm+diameter+chrome+steel+ball+bearing+g11
5mm x 1mm magnet:https://www.amazon.com/Multi-Use-Refrigerator-Magnets-Science-Crafts/dp/B073JZ42VJ/ref=sr_1_18_sspa?s=home-garden&ie=UTF8&qid=1516563013&sr=1-18-spons&keywords=5mm%2Bby%2B1mm%2Bsuper%2Bmagnet&psc=11
Use hot glue to attach the magnet.
