system
Given the following post, respond as if you're commenting on Reddit:
human
[How to train your dragon] What evolutionary purpose do the retractable teeth on a nightfury serve?
gpt
This feature probably developed to accommodate for the nightfury's unique fire breath.  The mouth of a nightfury is quite small compared to most other dragon species and it doesn't seem to open as wide.  If its teeth got in the way when the dragon breathes fire, it wouldn't be able to shoot the projectile fireballs that it's capable of producing without opening its mouth much wider.  Since the nightfury fires these projectiles while flying at a much higher speed than most other dragons are capable of, opening its mouth that wide would potentially cause problems and 'misfires' due to the large volume of air it would be involuntarily inhaling.