Controls can do more than enter or display data. They can work with data to produce temporary results, leaving the database unchanged. Suppose, for example, you want a control to divide unit prices by the number of items in a product unit, to show the price per item.
Calculated controls work by means of expressions. An expression combines arithmetic operators, fields, prebuilt formulas, and values to produce a sum. An expression can use data from a table, a query, or another control.
To create an expression, you would select the control, click the Properties button 
, and select the Data tab. Beside the Control Source property box, you would see three dots. Clicking those would open the Expression Builder dialog box.
In the dialog box, you could choose from lists of elements and enter specific values to perform a calculation. You’ll get a chance to try that in the practice session for this lesson.
