Here’s one I used to do in high school. Lick the pad of your index finger so that it gets good and sloppy with saliva. If it’s wet enough, you can hold a lit cigarette end-to-end in your fingers – the filter end’s against your thumb and the lit end’s against your index finger. You know, as if you’re demonstrating how long a cigarette is, except that there’s an actual cigarette between your fingers.
The lit end will crackle and pop like wild, and you can hold onto it until the saliva evaporates from the heat (this isn’t very long.) But the liquid will protect you from the heat, much like those guys who dip their hands in water and then straight into molten something-or-other without being hurt.
I assume the trick you’re referring to works the same way, using saliva as a barrier to the heat. But don’t go sticking lit cigars in your mouth just because I said so.
Before you criticize someone, walk a mile in their shoes. That way if they get mad, you’re already a mile away. And they don’t have any shoes.