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When you’re typing the name of a variable or class, just type a few letters and press CTRL+SPACE.  One of the following three things will happen:

• FlexBuilder will display a list of possible matches.  You can then use the arrow keys to find the variable or class you were looking for and press ENTER to auto-complete.
• FlexBuilder will auto-complete the name if there is only one possible match.
• You’ll hear a beep and nothing will happen.  This means that your variable is not defined or the class you’re looking for isn’t in the build path of the project.

Try to avoid manually typing things that can be auto-completed and you’ll almost never make a typo.