Task: Question Answering
Input (English): Context: In many cultures white is often associated with something positive. A white-collar job, for instance, is the kind of job many people look for, working with your brain and not your hands. White has a clean and pure image. That is why doctors, dentists, and nurses usually wear white uniforms. Babies are dressed in white at baptisms and brides wear white wedding gowns at weddings. White in these cases is the symbol of innocence or purity. Sometimes white is used in expressions that are not good. "Whitewash" is one such expression. At first, "whitewash" meant to paint over something with white paint to make it look better. However, it means something different today: to hide or to cover up mistakes or failures. A "white elephant" is another example of white used in a negative way. In ancient Thailand, a white elephant was regarded as a sacred animal, but it was very expensive to keep. The kings of those days presented a white elephant to the people they wanted to ruin. Once they received this holy, royal animal as a gift, they were not allowed to sell or kill it. Today, a "white elephant" means something that is big, useless, and unwanted. In America, when people want to get rid of their furniture or clothes, they often have a "white elephant sale". 
 Question: Which expression has an historical background?
Output (English): white elephant.