Refer to Class Method and Class Constant from outside of the class definition

A class object (like Ticket) has its own methods, its own state, its own identity. It doesn’t share these things with instances of itself.
我们可以把Class Method看作是class自己的Singleton Method. Class是制造object的工厂，工厂当然可以有自己的method啊！这些method是不给object的。

In referring to Ruby methods outside of code, it’s customary:
1, to refer to instance methods by naming the class in which they are defined, followed by a hash mark (#), and the name of the method;
2, to refer to class methods with a similar construct but using a period (.).
Sometimes you’ll see a double colon (::) instead of a period in the class method case.
我要求自己严格遵循：
Instance Method of a class写成：Ticket#show_price
Class Method of a class写成：Ticket::show_most_expensive

Class Constant
We refer to the constant from outside the class definition using a special path notation: a double colon (::).
class Ticket
      VENUES = ["Convention Center", "Fairgrounds", "Town Hall"]
end
$ puts "We've closed the class definition."
$ puts "So we have to use the path notation to reach the constant."
$ puts "The venues are:"
$ puts Ticket::VENUES

The double-colon notation pinpoints the constant VENUES inside the class known by the constant Ticket, and the list of venues is printed out.