system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: What Exactly Is Happening When A Program On Your Computer Is "Not Responding?"
gpt
Programs interact with the operating system using a "message queue". When you interact with a program (click the mouse, press a key) the OS sends a message to the program through the program's message queue. After the program handles the message, it responds to the OS saying the message was handled.

"Program not responding" happens when the OS is not receiving confirmation that the messages were handled. This can happen when the program is too busy to handle all the messages.

This shouldn't happen in a properly written program - the part of the program that handles the messages should be separate from the parts that do the heavy work, so if the program is busy it should still be responsive.