Improvisation is the activity of making or doing something not planned beforehand, using whatever can be found. Improvisation in the performing arts is a very spontaneous performance without specific or scripted preparation. The skills of improvisation can apply to many different faculties, across all artistic, scientific, physical, cognitive, academic, and non-academic disciplines; see Applied improvisation. Improvisation also exists outside the arts. Improvisation in engineering is to solve a problem with the tools and materials immediately at hand. Improvised weapons are often used by guerrillas, insurgents and criminals.