At about 5:30 in this video, you made a comment about your son complaining that you repeat stuff in your videos and how important it is to understand relationships.  Ironically, I didn't get how you related the two tables.  Were you able to do that just by copy/paste of the field "ProductCategoryID"? or did a relationship get created when Access recognized the two names were the same when you later dropped the subform into the parent form?

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Reply from Richard Rost:

You can create an EXPLICIT relationship between two tables in the Relationships Window.

If you don't, you will get what's called an AD HOC relationship which Access will create whenever you make a query or drop a subform into a form PROVIDED your fields are named the same, as you observed.