Hotheads are items that, when thrown, will slide across the ground releasing bursts of sparks and damaging your opponents. When hit with fire or electricity-based attacks, they grow bigger, dealing more knockback. However, in contrast to what its trophy says, there's an upper limit to how large the Hothead can become (roughly 3 times its original diameter or for a comparison, about Bowser's size).
It must be picked up and thrown to be activated. Once the Hothead lands on the ground or a platform, it will move along the surface in the direction it was thrown and periodically release sparks, which are almost 2 times as strong as a smash-thrown hothead of the same size. A Hothead can continue to grow after it is thrown. If it's thrown onto a stage, the Hothead will cling onto the stage's walls and surface. If, however, it lands on a platform, the Hothead will simply roll off until it finds solid ground (or just falls off the screen). To prevent this happening, throw the Hothead down on a solid platform, so that you don't waste getting easy damage. Hotheads eventually disappear in an amount of time inversely proportionate to their size (that is, bigger ones vanish faster).
A hothead will reach maximum power after it takes approximately 30%. But if the hothead is undamaged and hit by a fire or electric based attack that deals over 30% (such as Ike's Eruption), its power can exceed this limit and gain OHKO power. This can be taken to ludicrous extremes by knocking five Soccer Balls into a Hothead using Marth's Critical Hit (only in Training Mode is this plausible), causing the Hothead to deal 140% or more and becoming impossible to reflect.
In Super Mario World, Hotheads were enemies that appeared inside Wendy Koopa's Castle on World Six, Choco Island. This level largely comprised of stone blocks, which had Hotheads constantly moving around them. Unlike those in Super Smash Bros. Brawl, they were not enlarged by fire and they never disappeared. An interesting fact to note is that there were smaller versions of Hotheads, know as Li'l Sparkies. It is possible that the idea of them getting bigger in SSBB developed from combining the two foes.
A few hotheads were also featured in Burt the Bashful's Fort and Baby Bowser's castle in Super Mario World 2: Yoshi's Island.
An item that, when tossed, sticks to the terrain and moves, causing damage to those it touches. Whoever threw it won't take damage. It will grow larger if it's attacked, which ups its attack range. However, an increase in size corresponds to a decrease in the time it stays on the screen. A really big Hothead will take up the entire screen, and is quite a sight to behold.
Once thrown, this guy will move across the ground and damage anything--except you---in its past. You can make the Hothead bigger by hitting it with attacks, much to your opponents' dismay. If you make it really big and powerful, it just may carry you to victory!
SNES - Super Mario World (08/1991)
DS - Super Princess Peach (02/2006)
Oddly enough, even though Hotheads do fire damage, they cannot ignite Blast Boxes or idle Bob-ombs upon contact.
In game, Hotheads are actually rendered in 2D. However, in their trophy appearance they are in 3D.
If you attack a Hothead, it will emit non-damaging electricity.
Without the aid of items, Ike can create the strongest hothead out of any character by hitting it with a stage 8 Eruption.
The highest possible damage output known for a hothead without hacking is 274% when smash thrown and 426% when it sparks. For a hothead to become this strong, the situation with the five Soccer Balls and Marth's Critical Hit must be replicated, but after the Soccer Balls have sustained the maximum amount of damage.
A glitch called the 'Black Hole' glitch can be created by throwing the max amount of Hotheads into an enclosed area on a Stage Builder level. The Hotheads will continue to shock each other and will appear to 'suck' the enemy(s) towards them, they will also turn a dark purple (like that of the Shadow Bug's) or black.