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I received one of these as a gift, no receipt so I can't return it, and the marble is missing! Does anyone know how big it is supposed to be? I can manage to push the plastic dome in just a bit at the seam, so I'll be able to get the marble back in place with no problems as soon as I can find the right size and get it purchased!

It sounds like you received a defective ball. The distributor (Plasmart) will replace it - they are good to work with. You can contact them through the website at and explain what happened.