You see, there is an old toy shaped like a hammer that has a wooden rod sticking out of the top. Attached to this hammer-like object was a string with a ball on the end which has a hole the same diameter as the wooden rod. The goal with this toy was to get the ball to land right on that wooden rod in such a way that it goes into the hole.
Here they have replaced the hammer with the demon core, a metal hemisphere with a highly radioactive core in the center, and the ball with the other half of the metal sphere
A long time ago scientists were experimenting with the demon core and accidentally dropped the top half of the sphere onto the demon core, which made a large blue light and hit everyone in the room with radiation. Most of them fell ill and died.
Remember what the goal with the hammer was? That's what the kid is doing with the demon core. The joke here is that the top half of the demon core is about to land just right and most likely kill the child, this is a form of shitposting and dark humor.
