system
Given the following post, respond as if you're commenting on Reddit:
human
[LOTR] Elves and humans are different species, hoe are they capable of producing viable offspring?

Elrond, Arwen, and several other half-elven are the offspring of an elf/human coupling. They are clearly different species, though similar. How are they able to produce offspring that are able to reproduce? Is there a genetic alteration that occurs when they choose mortality?
gpt
These are not naturally evolved species in this universe.  Men and elves are both children of Iluvatar.  They were races created by a deity.

Apparently that deity either reused parts, inadvertently allowing crossbreeding, or intentionally made them compatible.