One head breathes gas, the other head lights it. Your job is to know which is which.
10 seconds sound clip from the How to Train Your Dragon movie soundboard.
You can hear this line at 00:37:20 in the Blu-ray version of the movie.
Quote context
[...]
- Now, a wet dragon head can't light its fire.
- The Hideous Zippleback is extra tricky.
- One head breathes gas, the other head lights it. Your job is to know which is which.
- Razor-sharp serrated teeth that inject venom for pre-digestion.
- Prefers ambush attack, crushing its victims...
- If that dragon shows either of his faces, I'm gonna... There!
[...]