From a completely silly but entirely deadpan debate about how, exactly, one would go about setting fire to the rain as described in the Adele song.
The rain obviously needs to contain or consist of some type of flammable liquid, and yet not be so volatile, and not disperse so much in vaporized form that an open flame would cause a chain reaction explosion. That would just explode the rain generating apparatus, and I argue that the song assumes a continuous event of fire setting and not a one-time incident.
The alternative option is that one sets fire to the rain after it has fallen, as one sets alight newspaper drenched in lighter fluid on the grill. However, I find this unlikely as well because the next line refers to the rain touching the singer's face in ways that grammatically hint at an ongoing action.
Now, rain could refer metaphorically not to the fall of liquid, but to a cascade of many small falling flaming objects. Or rather, small falling flammable objects, as setting fire implies a state transition caused by the singer - that is, one only sets fire to objects not yet aflame. This seems to me to be the most acceptable interpretation. Additionally, credence is given to the falling objects theory by the additional attribute that it is screaming out the addressee's name.
Here's my best theory. The rain refers poetically to a shower of small audio recording and playback devices upon which the singer has recorded the addressee's name. They have a thermally sensitive trigger. The electronics are well sealed. In particular, the battery is relatively well protected from the heat.
The insulated, nonflammable casing is wrapped in a wicking material and then doused in kerosene or similar. The singer places a large quantity of these soaked playback devices in a crane or other sturdy overhead construction apparatus, places the addressee nearby, lights the pile via a remote starter, touches the addressee's face, and then triggers the crane dump. The fire activates the thermal trigger. Devices playback the addressee's name. The singer hears it and cries on cue.
For added visual bonus, the casing only protects the electronics for so long, but eventually the battery overheats and explodes, which seems to match the drama of the piece.
I am unsure if this utilizes similar techniques as one might employ to set fire to the third bar, but if so, that would be an additional source to consider. That one is easy, though: enter two bars, do not set fire to them. Enter third bar, evacuate the third bar, put on appropriate personal protective equipment, set fire to the third bar.
Is the author of the rain song referencing the bar? We don't know! There are no details on where they acquired their methodology. This is why citations are important.