system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why do you see shapes/colors when you rub your eyes when they are closed?
gpt
You are stimulating your retina and causing the cells in it to send errant signals to your brain, which your brain interprets as bursts of color. 

The retina is a complicated structure with cells that basically take light and turn it into images your brain can understand. When you physically put pressure on those cells it can cause them to fire off some image signals to your brain without light. 

Those cells also require a lot of oxygen and when you put pressure on your eye you may also be cutting off their oxygen supply for a second or two. The brief lack of oxygen can also cause them to fire off image signals to your brain without light. That's why you often see colors and shapes whenever you get a bout of low blood pressure to your head (e.g., when you stand up too quickly).