It even comes with toy balls that your baby can drop into the toy and watch them spiral down to the ground below. (abckidsinc.com)
Dog toy balls come in all different sizes and materials ranging from hard rubber balls to foam balls and tennis balls. (pawster.com)
There are removable toy balls, but they don't really move and I think that is a plus as they don't distract the baby from his sleepy state. (pregnancymagazine.com)