What makes an object?
Here is my unofficial list:
The first item in this list is too restrictive. For example, you can think of your bank account as an object, but it is not made of material. (Although you and the bank may use paper and other material in keeping track of your account, your account exists independently of this material.) Although it is not material, your account has properties (a balance, an interest rate, an owner) and you can do things to it (deposit money, cancel it) and it can do things (charge for transactions, accumulate interest).
The last three items on the list seem clear enough. In fact, they have names:
This is a somewhat ordinary description of what an object is like. (This list comes from the book Object-oriented Analysis and Design, by Grady Booch, Addison-Wesley, 1994.) Do not be surprised if other notes and books have a different list. When you start writing object-oriented software you will find that this list will help you decide what your objects should be.
Consider a tube of four yellow tennis balls.