Putting an aggregate value in an UPDATE statement...

This statement:


UPDATE question_choices SET total_rows = count(*) from care_lesson;


returns the following error (which is well-documented):


ERROR:  cannot use aggregate function in UPDATE
LINE 1: UPDATE question_choices SET total_rows = count(*) from care_...


I wondered how I can put a row count from one table into another?  I  
can't figure it out.

Thanks very much,

John Gage