system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do computer screens ripple like water when you press on them?
gpt
LCD monitors / computer screens contain liquid crystals. These liquid crystals have properties of both a liquid (like water) and a solid (like a crystal). 

The reason for using liquid crystals is that (in combination with polarizing filters), by manipulating the amount of applied voltage it allows you to control the amount of light that is allowed to pass through the screen to your eyes in a given region of the screen. 

The backlight behind an LCD screen remains a relatively constant in brightness in most cases and it's the liquid crystals that determine whether to allow all the backlight through, only some of the backlight through, or none of the backlight through to dynamically control pixel brightness on the screen.