Our bodies undergo wear-and-tear, like the mailbox we had to fix last week. Only, in the case of our bodies, we don't yet have a way to fix or replace most of our parts. After a while, too many parts get worn down until they are no longer able to continue, and everything about us - the parts that allow us to think, the parts that allow us to feel - just stops.
Sadly, fixing our biology is harder than fixing mailboxes, and while we're learning more and more about how to do it every decade, we won't be able to fix everything for the foreseeable future.
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Too accurately:
So why aren't we trying harder to fix it? You see, son, society has been dealing with death for many thousands of years, and people have developed coping mechanisms to deal with its apparent inevitability. The majority of people simply don't think about it. Public figures have even rationalized death as a good thing, and boy have they said the darndest things. For example, I've recently read of someone comparing our bodies to legos in a fixed sandbox - as if people literally need to be disassembled before others can be born - and no one bats an eye! Another person pointed out that scientific progress would be slowed down without the older generation of scientists dying - sure, billions of non-scientists may die every few decades, but hey, science is a bit faster as a result!
The point is, society has created a culture of either ignoring or accepting death, and it shapes the very way we think. Sure, that culture might have been harmless back when we really had no chance of combating problems like old age, but now? The vast majority of researchers, of funders, don't even bother to think about what should be some of the most important problems of our generation. Until we have progressed to the point where it becomes blatantly obvious that the technology to combat old age exists, it is unlikely anyone will even notice it. In other words, we're all screwed and you were born a few hundred years too early.
> we're all screwed and you were born a few hundred years too early.
It's not a great option, but cryonics seems way better than nothing. (Under the assumptions that the preservation is reasonably high-quality, the storage facilities stay operational, humanity doesn't destroy itself, and advanced medical technology gets developed; see waitbutwhy.com/2016/03/cryonics.html )
An example of someone who wanted to live a long time but was genuinely screwed and was born a couple hundred years too early:
The rapid Progress true Science now makes, occasions my regretting sometimes that I was born so soon. It is impossible to imagine the Height to which may be carried, in a thousand years, the Power of Man over Matter. We may perhaps learn to deprive large Masses of their Gravity, and give them absolute Levity, for the sake of easy Transport. Agriculture may diminish its Labor and double its Produce; all Diseases may by sure means be prevented or cured, not excepting even that of Old Age, and our Lives lengthened at pleasure even beyond the antediluvian Standard. O that moral Science were in as fair a way of Improvement, that Men would cease to be Wolves to one another, and that human Beings would at length learn what they now improperly call Humanity! -Benjamin Franklin
So keep answering until you can't, and then say "I don't know". Curiosity is something to be fostered, not shutdown with lies.
When they're a little older and have more capacity to understand a fuller, more accurate, discussion is possible.
Only thing I can come up with is to wish everybody health and a long life.
The aspect of it that people are objecting to is that it does try to explain why. It says "Your father needs to go back in the box so we can make other people", which simply isn't true.
That said, I'm not a parent, and I realise it's easier to be idealistic in theory.
The lego analogy expresses the fact that we are not different from these other beings, and that we too must die so that life can continue. The death of an individual in this chain is always painful, but as part of the whole, it is just as it should be.
It's either that, or your child's future partner will have to deal with them losing faith in their parents, after so many things they were told turned out to be bullshit said to shut them up.