Straightening irons, straighteners, or flat irons work by breaking down the positive hydrogen bonds found in the hair's cortex, which cause hair to open, bend and become curly. Once the bonds are broken, hair is prevented from holding its original, natural form, though the hydrogen bonds can re-form if exposed to moisture. Straightening irons use mainly ceramic material for their plates. Low-end straighteners use a single layer of ceramic coating on the plates, whereas high-end straighteners use multiple layers or even 100% ceramic material. Some straightening irons are fitted with an automatic shut off feature to prevent fire accidents.