system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: How the finger light thing can tell how much oxygen is in your blood?

As the the title suggests, how is it that a little light they put on our finger when at the doctors/hospital/etc.. is able to tell us our blood oxygen levels?
gpt
In blood, the oxygen is carried by some molecules. 
It just so happens when the oxygen binds to those molecules they change color.  
So they shine a light, read the color and that figures out what proportion of those molecules have changed to that color.