My friend Florian had a theory that the Pokédex entries about Charmander dying when its flame goes out are a bit of a misconception.
He thought that if a healthy Charmander's flame is completely doused, it should actually be able to relight its tail within seconds. If it can't relight its tail, it's either because it can't produce enough of the gases needed to relight it, or that these gases have built up and can't escape. Both scenarios are deadly for a Charmander. In that sense, an unlit tail is more of an indicator or symptom than a cause.
But Florian said that he was 8 or so when the thought of it. He thinks it's probably not true, and we're not eager to find out through actual Charmanders...
- Juliana from @flor-and-jules
I think I answered this on an episode before, but I can reiterate it here!
Your friend Florian got very close from the truth, but it's a bit more complicated than that.
A charmander's flame is the main indicative of it's health, yes, but if their flame is doused by, let's say, the rain, they won't immediately die like some people may think.
The flame generated on their tail is a combination of their own ability to control fire and their diet. Charmanders are omnivores, but with a preference for dry foods and bugs in general. Part of what they consumed goes under a chemical reaction that basically turns what's left of it's food when it absorbs all the nutrients into a flammable gas. The excess of this gas is expelled from it's tail.
That's why "chonky charmanders" tend to have bigger flames. They eat more so they will produce more fuel.
But that gas is limited. If you starve your charmander or if something occurs to it's body that affects it's digestive system, the fire on it's tail will begin to consume necessary nutrients from the pokemon. The flame tends to get smaller once this starts to happen, so if you notice your charmander's flame is smaller than usual, go to a pokecenter.
But does that mean that you can douse the fire of a healthy charmander with no repercussion? No!
As stated, the gas they produce is limited. If they lose their flame for some natural reason, they can light it up again, but this will spend more fuel than usual. It doesn't kill the charmander outright, but it drains their fuel and, if they drain too much of it, they'll basically starve to death.