system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do DNA or cells grow a body? If all DNA or cells are "identical" then how to they know to grow a lung or a brain at the right time?

I have very limited knowledge of DNA and/or cells, and this question came from my father in law.

I tried googling it but everything came up extremely convulated and I wasn't sure if I got an answer.

If all cells are identical, how to they know what to "grow"? Can they sense what's around them?

Edit: thanks for the clarification everyone, my issue was thinking all the cells were identical and not realizing they take cues from their environment to know what to "be". Everything else - amazing info - was icing on the cake :)
gpt
Sensing what is around them is actually a crucial part of that, yes.  That's especially true for a newly-forming baby in the womb.

To simplify a bit, cells start out in a fetus as essentially blank cells called stem cells.  They are pre-programmed by evolution to undergo a process called differentiation which makes them take on a particular type of cell, like a skin cell, or a liver cell.  Then, those first few liver cells begin to multiply so your liver forms.  Cells have all kinds of intricate coding in their DNA to tell them what kind of cell they should differentiate into.

As for DNA, every cell in your body contains essentially the same copy of the DNA.  But they don't use the entire thing - not even close!  So, a liver cell only "reads" the parts of DNA that a liver cell needs to.

You've probably heard of stem-cells, those are these so-called "blanks".  Getting stem cells from non-fetus sources is kind of a huge deal, mostly because getting them from fetuses is highly controversial, and technology is rapidly improving in this regard.  If you treat them right, they can become any kind of cell, really.