Being "On fire" is a characteristic of certain Transformers, which can fit several definitions.
- Literally being perpetually engulfed in flames, for which the flames can be either real or magical.
- Being filled with enthusiasm, excited, or "on a roll." I.E. "Daniel finished all his homework in under an hour. He was On fire!"
- "On fire" was the name of a song by "The Con-Structs," an all trumpet six-piece band formed by the Constructicons. They performed several corinations, but broke apart when Optimus Prime shot them in the face.
It is the first meaning which holds most importance for some fans, because it deals with what looks to be a glaring logic error within Transformers canon. In this case, the flames surrounding the character have no apparent point of origin or fuel supply, and rarely seem to harm the engulfed individual or anything they interact with. Transformers who are "On fire" like The Fallen, Entropa, or Tricimus (Unicron's brother he totally forgot about until just now) are able to interact with items (for example handling a piece of paper or petting a kitten) with no apparent consequence when they so choose. Conversely, they may later use the same flames as a weapon, or to burn things around them when they need to look menacing and dangerous. They may also conveniently leave flaming footprints or skid marks when they do not normally, in the event the hero needs to follow them. This is often puzzling, as it implies a conscious ability on the part of the mechanism, to control when the fire "counts."
A wizard did it
Most Transformers who are “on fire” are written off as being somehow magical. This is largely a load of crap. “Magic” is often used as a blanket statement justifying, in essence, that the character in question doesn’t have to “make sense,” or answer to the “laws” of physics and logic, if they don’t feel like it right now. This is of course a silly thing to try to put into the otherwise serious and logical world of the Transformers.
Suspension of disbelief
Another popular explanation for the phenomenon is to tell the person asking to shut up. Be sure to inform them that it would be totally lame and distracting if they burned stuff on accident everywhere they went. Not only that, they would look stupid if they had no control over that sort of thing.