Whenever the program changes the value of a watched variable, the
  debugger suspends execution and displays the old and new values in the
  command view.

  To watch a variable (also known as setting a watchpoint on a variable),
  proceed as follows:

  1.  Monitor the variable as explained in the Monitoring a Variable
      topic.  The debugger puts a button in the Watched column of the
      monitor view whenever you monitor a variable.

  2.  Click on the button in the Watched column.  A filled-in button
      indicates that the watchpoint is set.

  To deactivate a watchpoint, clear its Watched button in the monitor
  view (by clicking on the button).  To activate a watchpoint, fill in
  its Watched button.

  The Accessing Static and Nonstatic (Automatic) Variables topic explains
  static and nonstatic (automatic) variables and how to access them.  The
  debugger deactivates a nonstatic watchpoint when execution moves out of
  (returns from) the variable's defining routine.  When a nonstatic
  variable is no longer active, its entry is dimmed in the monitor view
  and its Watched button is cleared.

  The debugger does not reactivate nonstatic watchpoints automatically if
  execution later returns to the variable's defining routine.  You must
  reactivate nonstatic watchpoints yourself.