Take apart a sealed bearing.
First remove the seal then drive a nail through the cage and pry the cage out with pliers. You now have a nice puzzle. How did they get the balls in there? Answer at the bottom. The first thing you notice is the balls are smaller and there are fewer of them typically 8 7/32 . If the bearing is not rusty it looks perfectly smooth even under a microscope. An ABEC 5 bearing is about 100 times more precise than a dura ace cone. A bent axle can wear out a sealed bearing in a few miles. Move all the balls to one side then the bearing will come apart now try to get them back in.