system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does red dot sights work?

you know, the red point to aim in games with guns. and in real life too.
if normally with a gun you need two separate points to know the gun is aiming the correct way (one bump near the trigger and another at the end of the cannon)
how does only one red point in a glass works for aiming??
gpt
The dot in a red or green dot sight is not actually a coloured point that's hovering in a fix spot in the air like an iron sight does.

Instead, the red dot you see is a reflection of a little red lamp that falls onto a spherical mirror which filters out only the red (or green) spectrum. This mirror is installed in a way that no matter what angle *you* look on it, the Red dot will always point towards the spot the bullet will hit. 

EDIT: That way the Red dot itself is virtual and basically only exists in your eyes, two people looking through the same sight at the same time from different angles would see the spots in different 3-dimensional places, but still pointed towards the target.