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The Flight of Dragons? (the book that The Flight of Dragons is based on)
They eat limestone to produce hydrogen gas for their fire breathing. I vaguely remeber a live-action TV show about some children finding a dragon in modren times that could be an adaption that had a similar explanation for fire breathing.
Edited by jormis29Flight of Dragons suggests a flint, and then rejects it in favour of chemical ignition, IIRC.
The dragons in the GrailQuest choose-your-own-adventure books have crystal and metal on their front teeth to ignite the methane generated by eating chestnut leaves.
It looks like variants of this idea are quite common, I'm afraid...
Edited by DaibhidC

I read it when I was a kid and it's always bugged me. It has dragons who breath gas and strike something like a flint on their tongue/back of mouth or throat to make a spark and that's how they breath / make fire.