- programming:
  1) define classes
  2) use classes & objs to accomplish tasks
- message: instruct class/obj to perform task
- must program class/obj to process msg
  - method: instructions to perform a task
    - class: defined for class  } more
    - instance: defined for obj } below
  - argument: value passed to obj in association w/ msg
  - msg name sent must match some method def
- instance methods: pertain to individual instance/obj
  - one-way msg
    - obj performs msg op
    - obj does not communicate back to msg sender
  - two-way msg: obj returns information to sender
    - can be data
    can be success/fail status
- class methods: pertain to all instances of a class
  - deal w/ collective info about all instances of a class

Quick Check:
1.
                     _____________
                    |             |
   deposit(amount)  | a : Account |
------------------->| ----------- |
                    |             |
  withdraw(amount)  |             |
------------------->|             |
                    |_____________|

2. Instance.