I was reading various questions about whether or not grease is flammable, which got me thinking about other flammable materials, so I'm asking what qualifies a material as flammable or not and how spells that can ignite flammable objects interact like burning hands.
For example if your party is in a forest during the summer and your fighting goblins what happens when you cast burning hands, do you start a forest fire which is likely what would happen in reality if you created a 15ft gout of flames in a dry forest.
Another example would be if your party is fighting cultists who all wear robes do the robes catch fire when hit with burning hands or other spells capable of igniting things, and if the robes do catch fire they begin taking burning damage until the robes are destroyed or extinguished.
One final example I have is what would happen if you casted burning hands on a cask of liquor that would likely be flammable which is normal for liquors that are above 100 proof(50% alcohol content), or if you casted burning hands in a alchemists lab where their would likely be many different flammable liquids.
I'm mostly wondering this to find some new and fun ways to defeat enemies early on such as somehow dousing a group of orcs in a strong liquor and lighting them up with a simple burning hands spell.
Best Answer
All editions of D&D (and indeed, almost all RPGs) are written with us and our knowledge of the world as the baseline, only adding to and altering that where it needs to and says so.
So when the game says "flammable objects", it's relying on the words "flammable" and "objects" to have their dictionary meanings, and the set of flammable objects is the same as in the real world except where the game specifically adds or removes objects from that set. Thus, a summer forest may* contain dry tinder that may* catch on fire, and normal cotton robes† may catch on fire. A cask of high-proof liquor is potentially flammable, just like in real life, but also just like in real life, it's not going to be lit by a momentary gout of flame because solid wood staves don't light instantly — to blow up a volatile liquid in a wooden cask, you have to expose it to intense heat for more than a few seconds.
* I say "may" twice, because unless you previously establish that the target is standing in dry tinder, it's only possible that the burning hands fan also hits something that could catch on fire; and because even if some tinder catches on fire, you're not guaranteed that the whole forest goes up in flames. This is where your discretion as a DM comes in: decide what a sensible chance is, and ask the dice.
† Speaking of things altered by the game rules though: if a flammable object is worn or carried by someone, it does not automatically catch on fire just because burning hands says flammable objects catch fire. Worn and carried objects always grant a saving throw against that kind of outcome.