A controlled explosion is a way of tower block demolition, as well as the demolition of other tall structures. It is now widely accepted as the safest form of demolition in tower blocks. For safety, the building is completely stripped of inner walls, windows and balcony railings. This preparation can take weeks or months. Electricity, gas and heat supplies are discontinued. Fencing is erected around the site to prevent unauthorised persons from entering. This is sometimes guarded at night. Explosives are strategically placed within the shell of the building, and these are detonated in a rhythmic fashion so that the structure will collapse in a particular way. Most of the time, the explosives are detonated to make the tower block collapse in on itself, reducing the risk of flying debris.