Currency crises can be especially destructive to small open economies or bigger, but not sufficiently stable ones. Governments often take on the role of fending off such attacks by satisfying the excess demand for a given currency using the country's own currency reserves or its foreign reserves (usually in the United States dollar, Euro or Pound sterling). Currency crises have large, measurable costs on an economy, but the ability to predict the timing and magnitude of crises is limited by theoretical understanding of the complex interactions between macroeconomic fundamentals, investor expectations, and government policy.[4] A currency crisis may also have political implications for those in power.