Experiment 14: What's in a Computer Mouse Ball?

So mouse balls, the kind inside computer mice, not real mice (!!!), are very heavy, but what inside them makes them heavy?  To answer that question, I cut one of them open with a box cutter.  Then, I scientifically observed the contents, and came to the conclusion that mouse balls have a steel ball bearing inside them to give them their weight.  I observed that the metal sphere inside the rubber outer coating was 28 grams in weight and was dull silvery in color.

The sphere was magnetic as well.  I then calculated the diameter with a pair of calipers, then halved this to get the radius.  By cubing this, then multiplying it by 4/3pi, I got its volume in cubic centimeters.  Then, by dividing its weight by its volume, I got approximately 7.03 grams/cm^3 as its density.  This seems a little bit light for steel, but the material was magnetic.  Select other elements are magnetic, but most elements with similar densities (~7 g/cm^3) don't make sense for a mouse ball.  Who makes trackballs out of neodymium?  Therefore, without other data, it is reasonable to conlude that it is indeed steel inside a computer mouse ball.