How Are Baby Bones Made?

Probably the biggest question is: How were these two born?
Did the humans that went to the Underground, who also died, turn into skeletons when they died and their SOUL was taken?
Eh...I don't think so. There were coffins, containing the bodies in the castle. So that's not it.
Did some bones just magically come to life?
Or...maybe some murderous humans threw bodies down into the underground which landed in the Garbage Dumps, then possessed by either ghosts or monster dust which then lived on in the skeleton bodies?
Or is there some magical skeleton biology that we don't know about?
GAH! HOW ARE THESE THINGS MADE?!?!?!?
I, THE GREAT ZORA, DEMAND AN ANSWER!!!!!!

Sans and Papyrus: Undertale/Toby Fox©