Many times the data you need to enter in one table can be found in another table. In the Northwind database, for example, if you add a new product, you must assign it to a category. Rather than make you open the Categories table separately and find the value you need, Access lets you display those values in a list. You can also create your own list, rather than drawing values from an existing source.
Open a table in Design view.
Click the field to define as a lookup field.
Click the Data Type ...