As is the case with all metallic items, aluminium foil reacts to being microwaved. This is due to the effect of electric fields of the microwaves causing a build up of charge to form between the sharp points in the aluminium; if enough charge accumulates it will discharge to a different place on the foil, creating a spark (i.e., arcing). The design of modern microwave ovens has been corrected so microwave energy cannot be reflected back into the magnetron, and aluminium packages designed for microwave heating are available.