Why are American bills or paper money unlike that in many other countries?

One of the first things a visitor does upon arrival in another country is to get to know the local money. American money is unlike the money used in many other countries, since the bills, or paper money, are all the same colour and size no matter how much it is. Visitors should be careful to look at the bill before giving it out. Besides, new bills often stick together.
    The coins of every country are difficult to learn at first. American coins are represented by the cent sign. Also the size of American coins do not show the value. For example, the 10-cent coin is smaller than the 5-cent coin, although it is worth twice as much. All United States coins are silver coloured, except for the "penny" (one-cent piece) , Which is a brown coin. Here are more details about Ameican coins.