Steel ball in 3D printed sphere (having 30 mm diameter).

Use a steel (or glass or wooden) ball with 8 mm diameter and wait until half of the sphere is printed, then put the ball carefully in the half sphere (while the 3D printer is running). The sphere that you will obtain will have a support material inside. This has to be cleaned at the end (quite difficult) :-(

Attribution - Creative Commons

3D, sphere, ball