system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: How does a single cell multiply into exact copies of itself yet somehow knows to start building into different body parts? They are all in the same environment with the same DNA yet behave differently.
gpt
Each cell has all the DNA, however based on external signals the cell activates certain enzymes that read specific DNA to become whichever organ. 

Theoretically, you can introduce a reading enzyme from a liver cell to a heart cell and that enzyme will read the heart cell DNA to make liver stuff.