Hi Richard, you may consider making a video for this.  It's a simple problem that stumped everyone at another forum I just saw.  

On a form, there are two text boxes with *calculated* numeric values as their control sources. Let say one has "=4/4", which equals to one, and another has "=2*5" which equals to 10. There is another text box for user entry. If the entry is outside the range of the two calculated fields, the user gets an error message.  So a BeforeUpdate event procedure is made:

Private Sub UserEntry_BeforeUpdate(Cancel As Integer)
    If Me!UserEntry < Me!CalcF1 Or Me!UserEntry > Me!CalcF2 Then
        MsgBox "Out of range."
        Cancel = True
    End If
End Sub

And yet, when the user enters 5, which is in range (between 1 and 10), he still gets the "out of range" error.

It turns out the UserEntry is treated as a STRING, and that's why the comparison doesn't work.

UserEntry must be converted to numeric. For instance: If Val(Me!UserEntry)< ...

Here is the forum thread where I saw this:
https://www.accessforums.net/showthread.php?t=85869

It's an interesting, simple, yet important little issue that your students may benefit from.