system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the biology behind the chameleon' s ability to alter it's skin colour?
gpt
Just to get it out of the way. Chameleons only occasionally change color to blend in, mostly it's to communicate with other chameleons.  
Chameleons have tiny crystals in their skin.   
These crystals cause light to bounce around and get interfered with.   
They the have muscles that control how far apart the crystals are. 
This changes how the light gets bounced around and interfered with, changing the color.   
  
Incidentally this is different from how the octopus and the cuttlefish change color. 
They have many different color spots, and have muscle to cover or uncover them, thus changing the overall color.