Burny Stuff is the second upgrade for Path 2 for the Mortar Tower in BTD5, or the second upgrade of Path 3 in BTD6. This upgrade adds napalm that sets Bloons on fire for 6 seconds. Every 2 seconds, one layer gets popped. If a Bloon gets hit while the Burny Stuff upgrade is activated, the Bloon will have monkey napalm and be on fire for 6 seconds.
In BTD6, the bloons are set on fire for only 3 seconds, damaging one layer every 1.5 seconds, or every 1.0s seconds with the Extra Burny Stuff Monkey Knowledge perk. In addition, Burny Stuff can no longer affect Black Bloon properties without special upgrades such as Heavy Shells.
In both BTD5 and BTD6, it costs $430 on Easy, $500 on Medium, $540 on Hard and $600 on Impoppable.
The tower upgraded to the Burny Stuff upgrade looks like the original Mortar Tower, except with a fiery design on it. In BTD6, not only does it gain a fiery design, the hat also has a fiery design too.
With this upgrade, the Mortar Tower can pop Black and Zebra Bloons, they will be not popped by the explosion, but they will be popped by the napalm. However, in BTD6 the Heavy Shells upgrade is required.
Bloons can be affected by ice and napalm at the same time.
This upgrade makes the Mortar Tower act similarly to a Molotov because when somebody throws it, it explodes and gets napalm on somewhere.
The explosion turns orange after a player upgraded the Mortar Tower to Burny Stuff shown below.
In BTD6, a Heavy Shells with Burny Stuff could not affect Black Bloons until Version 7.0, when it got fixed.
In BTD6, the description for Burny Stuff is not completely accurate. Although it claims that bloons are set on fire on "each hit", they are only set on fire if bloons are damaged. So a Black Bloon can be "hit" by a Mortar Monkey, but it won't be affected by it.