Whether a cryptocurrency can become fiat money or even replace what is issued by the central government has always been a hot topic in the cryptocurrency world and major traditional financial institutions. On June 9th, El Salvador passed a bill under the efforts of the young President Nayib Bukele to list Bitcoin as the country's fiat money, which once again sparked intense discussions around the world. Why did El Salvador, one of the poorest countries in the world, choose Bitcoin as its fiat money? What is the relationship between currency, fiat money, and cryptocurrency?