Headlights are one of the most important parts of the automobile exterior. Glass headlights were standard between the 1950s and 1970s, but most modern vehicles use resin headlights, which are light and durable. Most resin headlights are made of plastic, which can be easily processed into various shapes. Today, headlights are not simply lights, but key parts of the design of a vehicle that come in a variety of forms. However, it is also true that there are various problems with plastic headlights which do not occur in glass headlights. The most serious of those problems is "yellowing", when the headlight begins to turn yellow. It sometimes takes only three years for the headlights of a new car to begin yellowing. The auto detailing industry has long sought ways to deal with this, but the causes of deterioration are complex, and one misstep during headlight cleaning can cause various other problems. It remains challenging to handle yellowing reliably.