making macro run auto when a third part result is met

hi
I have some doubts with this code

 Private Sub Worksheet_Open()
CopyRange
End Sub

Sub CopyRange()
Dim X, MyString
X = Range("N25")
Range("C2").Copy Destination:=Range("S2")
End Sub

this macro only run manualy and i want it to run auto when i get a third part result
f.e. when cell A8=5, run auto the macro

thanks in advance