Flamestrike
Legend
You underestimate the scientific spirit of my PCs! Trial and error, my friends. Trial and error.
P.S. On a more serious note, metagame knowledge is important too, which is why the Player's Handbook exists. Everything in it is metagame knowledge, and that knowledge empowers the players. Without metagame knowledge, the players cannot effectively act in-game except through that very trial and error which I poke fun at above. Imagine how much fun the game would be if you had to deduce over time, by trial and error, what the effect of Great Weapon Fighting was. Not very fun!
There is a difference between knowing the rules and playing by the spirit of the rules.
Im a lawyer IRL. Legislation always contains absurd results if you argue it 'RAW'. Thats why we have the Judiciary. The Judge doesnt just read laws according to the rules as written, sans any context - they look at the rules within the context of the legislation, and of other legislation. They consider how the law should work, and what the law is for. Importantly, they also look to Hansard, readings of the bill, parliamentary debates etc (to see what the legislature's intent was for drafting the law, and what the legislature was intending on the law to do).
And then of course you have the Judge themselves own bias and interpretation (like it or not, we're all human).
I mean; does the 'Right to Bear Arms' mean:

Or something else?
It's why I've never understood people who scream RAW at me like its some kind of objective truth that exists as 'a thing in and of itself'. It doesnt, and logically cannot exist without context or interpretation. In other words it is impossible to take the RAI out of the RAW, so I have no idea why people even try.
Luckily, the game (like the law) includes a referee, and that referee gets the final call on rules interpretations (among many other responsibilities).
By RAW, hit points represent 'luck and the will to live' in addition to health and vitality. Lava deals a lot of damage because you have to be pretty darn unlucky to fall into a vat of lava. There is also the heat of course

A player that willingly cuts his own throat, or jumps into a river of lava has either run out of luck, or lost the will to live. Accordingly he dies.
Hopefully the look on my face will tell the player in question that such an idea is stupid before he attempts such a thing, and in any event, I'll only have to deal with such a situation once.