An economic bubble or asset bubble is trade in an asset at a price or price range that strongly exceeds the asset's intrinsic value. It could also be described as a situation in which asset prices appear to be based on ＿＿（18）＿＿ or inconsistent views about the future. Asset bubbles date back as far as the 1600s and ＿＿（19）＿＿ now widely regarded as a recurrent feature of modern economic history. Because it is often difficult to observe intrinsic values in real-life markets, bubbles are often conclusively identified only in ＿＿（20）＿＿ , once a sudden drop in prices has occurred. Such a drop is known as a crash or a bubble burst. Both the boom and the burst phases of the bubble are examples of a positive feedback mechanism, ＿＿（21）＿＿ the negative feedback mechanism that determines the equilibrium price under normal market circumstances. Prices in an economic bubble can fluctuate ＿＿（22）＿＿ , and become impossible to predict from supply and demand alone.