The class is much like a cookie cutter for objects. Once the program is running and you create an object, the object has the members it will have until it is destroyed. You can change the value of the members (such as changing the height member of a Rectangle object from 10 to 20 or changing the balance member of the CheckingAccount object from 100 to 100,000,000) but you can't add or remove members. While your program is running, you can't add a depth member to your Rectangle class. The only way you can make changes is by changing the code and recompiling. But then when you run the program, all your Rectangle objects will have the new member.