Add fire. Lava can cause it, and you can /giveme fire:basic_flame.

mods/default/init.lua

@@ -1009,7 +1009,7 @@ minetest.register_node("default:water_flowing", {
 		{image="default_water.png", backface_culling=false},
 		{image="default_water.png", backface_culling=true},
 	},
-	groups = {water=3, liquid=3},
+	groups = {water=3, liquid=3, puts_out_fire=1},
 })
 
 minetest.register_node("default:water_source", {
@@ -1032,7 +1032,7 @@ minetest.register_node("default:water_source", {
 		-- New-style water source material (mostly unused)
 		{image="default_water.png", backface_culling=false},
 	},
-	groups = {water=3, liquid=3},
+	groups = {water=3, liquid=3, puts_out_fire=1},
 })
 
 minetest.register_node("default:lava_flowing", {
@@ -1056,7 +1056,7 @@ minetest.register_node("default:lava_flowing", {
 		{image="default_lava.png", backface_culling=false},
 		{image="default_lava.png", backface_culling=true},
 	},
-	groups = {lava=3, liquid=2, hot=3, igniter=3},
+	groups = {lava=3, liquid=2, hot=3, igniter=2},
 })
 
 minetest.register_node("default:lava_source", {
@@ -1080,7 +1080,7 @@ minetest.register_node("default:lava_source", {
 		-- New-style lava source material (mostly unused)
 		{image="default_lava.png", backface_culling=false},
 	},
-	groups = {lava=3, liquid=2, hot=3, igniter=3},
+	groups = {lava=3, liquid=2, hot=3, igniter=2},
 })
 
 minetest.register_node("default:torch", {
@@ -1100,7 +1100,7 @@ minetest.register_node("default:torch", {
 		wall_bottom = {-0.1, -0.5, -0.1, 0.1, -0.5+0.6, 0.1},
 		wall_side = {-0.5, -0.3, -0.1, -0.5+0.3, 0.3, 0.1},
 	},
-	groups = {choppy=2,dig_immediate=3},
+	groups = {choppy=2,dig_immediate=3,flammable=1},
 	legacy_wallmounted = true,
 	sounds = default.node_sound_defaults(),
 })

mods/fire/README.txt

@@ -0,0 +1,30 @@
+Minetest 0.4 mod: fire
+======================
+
+License of source code:
+-----------------------
+Copyright (C) 2012 Perttu Ahola (celeron55) <celeron55@gmail.com>
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 2 of the License, or
+(at your option) any later version.
+
+http://www.gnu.org/licenses/gpl-2.0.html
+
+License of media (textures and sounds)
+--------------------------------------
+Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0)                                 
+http://creativecommons.org/licenses/by-sa/3.0/
+
+Authors of media files
+-----------------------
+Everything not listed in here:
+Copyright (C) 2012 Perttu Ahola (celeron55) <celeron55@gmail.com>
+
+fire_small.ogg sampled from:
+  http://www.freesound.org/people/dobroide/sounds/4211/
+
+fire_large.ogg sampled from:
+  http://www.freesound.org/people/Dynamicell/sounds/17548/
+

mods/fire/init.lua

@@ -0,0 +1,188 @@
+-- minetest/fire/init.lua
+
+minetest.register_node("fire:basic_flame", {
+	description = "Fire",
+	drawtype = "glasslike",
+	tile_images = {"fire_basic_flame.png"},
+	light_source = 14,
+	groups = {igniter=3,dig_immediate=3},
+	drop = '',
+	walkable = false,
+})
+
+local fire = {}
+fire.D = 6
+-- key: position hash of low corner of area
+-- value: {handle=sound handle, name=sound name}
+fire.sounds = {}
+
+function fire.get_area_p0p1(pos)
+	local p0 = {
+		x=math.floor(pos.x/fire.D)*fire.D,
+		y=math.floor(pos.y/fire.D)*fire.D,
+		z=math.floor(pos.z/fire.D)*fire.D,
+	}
+	local p1 = {
+		x=p0.x+fire.D-1,
+		y=p0.y+fire.D-1,
+		z=p0.z+fire.D-1
+	}
+	return p0, p1
+end
+
+function fire.update_sounds_around(pos)
+	local p0, p1 = fire.get_area_p0p1(pos)
+	local cp = {x=(p0.x+p1.x)/2, y=(p0.y+p1.y)/2, z=(p0.z+p1.z)/2}
+	local flames_p = minetest.env:find_nodes_in_area(p0, p1, {"fire:basic_flame"})
+	--print("number of flames at "..minetest.pos_to_string(p0).."/"
+	--		..minetest.pos_to_string(p1)..": "..#flames_p)
+	local should_have_sound = (#flames_p > 0)
+	local wanted_sound = nil
+	if #flames_p >= 9 then
+		wanted_sound = {name="fire_large", gain=1.5}
+	elseif #flames_p > 0 then
+		wanted_sound = {name="fire_small", gain=1.5}
+	end
+	local p0_hash = minetest.hash_node_position(p0)
+	local sound = fire.sounds[p0_hash]
+	if not sound then
+		if should_have_sound then
+			fire.sounds[p0_hash] = {
+				handle = minetest.sound_play(wanted_sound, {pos=cp, loop=true}),
+				name = wanted_sound.name,
+			}
+		end
+	else
+		if not wanted_sound then
+			minetest.sound_stop(sound.handle)
+			fire.sounds[p0_hash] = nil
+		elseif sound.name ~= wanted_sound.name then
+			minetest.sound_stop(sound.handle)
+			fire.sounds[p0_hash] = {
+				handle = minetest.sound_play(wanted_sound, {pos=cp, loop=true}),
+				name = wanted_sound.name,
+			}
+		end
+	end
+end
+
+function fire.on_flame_add_at(pos)
+	--print("flame added at "..minetest.pos_to_string(pos))
+	fire.update_sounds_around(pos)
+end
+
+function fire.on_flame_remove_at(pos)
+	--print("flame removed at "..minetest.pos_to_string(pos))
+	fire.update_sounds_around(pos)
+end
+
+function fire.find_pos_for_flame_around(pos)
+	return minetest.env:find_node_near(pos, 1, {"air"})
+end
+
+function fire.flame_should_extinguish(pos)
+	--return minetest.env:find_node_near(pos, 1, {"group:puts_out_fire"})
+	local p0 = {x=pos.x-2, y=pos.y, z=pos.z-2}
+	local p1 = {x=pos.x+2, y=pos.y, z=pos.z+2}
+	local ps = minetest.env:find_nodes_in_area(p0, p1, {"group:puts_out_fire"})
+	return (#ps ~= 0)
+end
+
+minetest.register_on_placenode(function(pos, newnode, placer)
+	if newnode.name == "fire:basic_flame" then
+		fire.on_flame_add_at(pos)
+	end
+end)
+
+minetest.register_on_dignode(function(pos, oldnode, digger)
+	if oldnode.name == "fire:basic_flame" then
+		fire.on_flame_remove_at(pos)
+	end
+end)
+
+-- Ignite neighboring nodes
+minetest.register_abm({
+	nodenames = {"group:flammable"},
+	neighbors = {"group:igniter"},
+	interval = 1,
+	chance = 2,
+	action = function(p0, node, _, _)
+		-- If there is water or stuff like that around flame, don't ignite
+		if fire.flame_should_extinguish(p0) then
+			return
+		end
+		local p = fire.find_pos_for_flame_around(p0)
+		if p then
+			minetest.env:set_node(p, {name="fire:basic_flame"})
+			fire.on_flame_add_at(p)
+		end
+	end,
+})
+
+-- Rarely ignite things from far
+minetest.register_abm({
+	nodenames = {"group:igniter"},
+	neighbors = {"air"},
+	interval = 2,
+	chance = 10,
+	action = function(p0, node, _, _)
+		local reg = minetest.registered_nodes[node.name]
+		if not reg or not reg.groups.igniter or reg.groups.igniter < 2 then
+			return
+		end
+		local d = reg.groups.igniter
+		local p = minetest.env:find_node_near(p0, d, {"group:flammable"})
+		if p then
+			-- If there is water or stuff like that around flame, don't ignite
+			if fire.flame_should_extinguish(p) then
+				return
+			end
+			local p2 = fire.find_pos_for_flame_around(p)
+			if p2 then
+				minetest.env:set_node(p2, {name="fire:basic_flame"})
+				fire.on_flame_add_at(p2)
+			end
+		end
+	end,
+})
+
+-- Remove flammable nodes and flame
+minetest.register_abm({
+	nodenames = {"fire:basic_flame"},
+	interval = 1,
+	chance = 2,
+	action = function(p0, node, _, _)
+		-- If there is water or stuff like that around flame, remove flame
+		if fire.flame_should_extinguish(p0) then
+			minetest.env:remove_node(p0)
+			fire.on_flame_remove_at(p0)
+			return
+		end
+		-- Make the following things rarer
+		if math.random(1,3) == 1 then
+			return
+		end
+		-- If there are no flammable nodes around flame, remove flame
+		if not minetest.env:find_node_near(p0, 1, {"group:flammable"}) then
+			minetest.env:remove_node(p0)
+			fire.on_flame_remove_at(p0)
+			return
+		end
+		if math.random(1,4) == 1 then
+			-- remove a flammable node around flame
+			local p = minetest.env:find_node_near(p0, 1, {"group:flammable"})
+			if p then
+				-- If there is water or stuff like that around flame, don't remove
+				if fire.flame_should_extinguish(p0) then
+					return
+				end
+				minetest.env:remove_node(p)
+			end
+		else
+			-- remove flame
+			minetest.env:remove_node(p0)
+			fire.on_flame_remove_at(p0)
+		end
+	end,
+})
+