Take a torch, like a wick wrapped around a stick doused in kerosene. Light it on fire. Now stick it in your mouth.
If you do it without hesitation, and close your mouth completely, the oxygen supply will be cut off and the torch will go out. If you hesitate, you might get burned. I practically make a living at this. See busking.

There is a secret to fire-eating. You know that little piece of flesh that hangs down in the back of your mouth, it's called your uvula? Well, in most people, the uvula just hangs there. I, however, have a prehensile uvula. When I insert the flaming torch into my mouth, my uvula reaches out, wraps around the torch, and extinguishes it. And that's how fire-eating is done.

I told that story once to a lady who looked at me and said, "When did you learn you could do that?"

She was from Texas.