The hood serves a very important purpose of covering and protecting the engine. In terms of sheer size, it is easier one of the largest single parts on the car. It will have to be replaced if it is damaged or dented. This may prevent the piece from closing properly and it may then pop open at speed greatly limiting visibility. Most production hoods are simply made or stamped steel or aluminum. The aluminum parts are much lightly meaning they are easy to open and are often found on performance cars. Aftermarket or custom part may be made out of carbon fiber or fiberglass. Sometimes they will have ram-air cutouts or an open section to clear air cleaners.
The hood or bonnet has evolved over the years with the styling and engineering changes of the cars. They are generally square or rectangular shape but they open in different ways. Older cars often had a two piece part that opened down the middle. Other styles are the clamshell design, the forward-opening and the side-opening hoods. It is attached to the car only by hinges and springs. Once it is opened it is held in place either by a metal prop rod or by gas struts. The gas struts also assist in opening and are usually found on newer, more expensive cars.
Replacing a hood is one of the easier things to do involving the large sheet-metal parts on a car. Be sure to have all the tools you need and it is a good idea to have someone to help you hold or carry the part. Here are some basic instructions on how to change the bonnet.