system
Given the following post, respond as if you're commenting on Reddit:
human
[Lord of the Rings] If an Elf and a Human bore a child together, would the child live forever?

Considering Elves are immortal and men aren't...
gpt
You are asking about the Halfelven, and the basic answer is that they have to choose whether they are counted among Elves or Men. When the Man Tuor married Idril Celebrindal, daughter of Turgon, High King of the Noldor, there son Eärendil was required by various circumstances to accept the life of the Eldar, though his heart lay more with Men. The sons of Eärendil were able to choose freely, however. Elros chose mortality for himself and his descendants, and became the first King of Numenor, distant ancestor to Aragorn. His brother Elrond chose to become one of the Elves. His children faced the choice as well, though they were permitted to delay the decision until Elrond left Middle-earth. To remain immortal, they would have to leave for Valinor with him; staying behind meant accepting mortality.

Now, you may have noticed that Elrond's descendants got to choose, but Elros' descendants had the choice made for them. This has because mortality is a Gift to Men from God himself. The Elves are bound to the world, never to leave it until its ending. Men dwell in it for a time, but then get to leave it for whatever lies beyond. Nothing short of God himself, not even the Powers that rule this world, can stand in the way of this Gift. In receiving it, Elros entitled his children to receive it, as well.