system
Given the following post, respond as if you're commenting on Reddit:
human
[How To Train Your Dragon] What are the evolutionary advantage of retractable teeth?

As far as I know the Night Fury is the only species that retracts its teeth. My first thought was that it might protect the teeth against the fire breath but it doesn't seem to be an issue with other dragons.
gpt
The Night Fury projects a far more concussive blast than other species of Dragons, which seems to detonate upon contact with anything. The retractable teeth are almost definitely a safety mechanism, to prevent the blast from exploding inside it's own skull.