Fire is both a hazardous and a necessary object found in certain World of Goo. It is entirely dangerous, however, like dangling over dark expanses, it is necessary to complete many levels. Fire is not affected by physics or other stage elements, save for speed. Fire destroys any Goo it touches and can travel along the legs of flammable Goo.
Explosive types of Goo will be lit up for a second before exploding upon contact with fire, while most Goos will merely pop. Fire remains stationary in a level, neither spreading or shrinking, until a Goo touches it. This will cause the fire to spread throughout the structure this Goo is attached to (so long as the legs of the Goo the structure is made out of are flammable). The source fire remains stationary and cannot be extinguished.
Fire is mostly used for setting structures aflame; this in turn can be used to lower objects, remove barriers, and detonate Bombs, Sticky Bombs, and other explosives.