system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can't only certain pixels light up on a phone, for example only the pixels necessary for displaying the time, why does every pixel need to light on the screen just to see the time?
gpt
Depends on the display technology. 

In an LCD, pixels don't light up. There's one big light behind all of them, and the pixels are basically chemical "shutters" to block it. 

On an OLED screen, individual dots do light up, so they can really do what you're describing.