system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When looking at a glass, why are we able to choose between seeing our reflection and seeing through it.
gpt
You can focus your eye to see a sharp image at different distances by adjusting the lens.

Try holding your finger at arm's length before your face. Focus on the finger and the background is blurred, focus on the background and the finger is blurred.

Exactly the same thing happens with your reflection, which is in focus at twice the distance between you and the glass, while whatever is outside it usually further away.