Why doesn't "% Complete" change the "Finish Date" for a Task. And how can I do that?

For example:
Task A Work=20 hours Resource=John
Start Date = Today
End Date Calculated to be 12/14/09

Now say tomorrow I check with John, and he is way ahead of schedule; 50% Complete (10 hours left). So I update the % Complete=50 for
Task A, but, the End Date stays the same!

Obviously, what I want to do is have the end date move up - to refect that we are ahead of schedule (and John will finish early, thus starting his next task sooner. How can I do this in MS Project?

Thanks very much!