Calculated controls use expressions as their control sources. To create a calculated control, you must first add an unbound control to a report. An expression must be preceded by an equal sign (=); an example of a report expression is =Sum([Freight]). This expression, if placed in the report footer, totals the contents of the Freight control for all detail records in the report. You can build an expression by typing it directly into the control source or by using the Expression Builder, covered in Hour 16, "Power Form Techniques."