forcing variable declaration with option explicit

When Option Explicit On or Option Explicit appears in a file, you must explicitly declare all variables by using the Dim or ReDim statements. If you try to use an undeclared variable name, an error occurs at compile time. The Option Explicit Off statement allows implicit declaration of variables.
On
Optional. Enables Option Explicit checking. If On or Off is not specified, the default is On.
 
Off
Optional. Disables Option Explicit checking.

Option Explicit On

Option Explicit On

Module Module1
    Sub Main()

        Dim thisVar As Integer
        thisVar = 10
        ' The following assignment produces a COMPILER ERROR because 
        ' the variable is not declared and Option Explicit is On.
        thisInt = 10 ' causes ERROR

    End Sub
End Module

Option Explicit Off

Option Explicit Off

Module Module1
    Sub Main()

        Dim thisVar As Integer
        thisVar = 10
        thisInt = 10 ' NO ERROR

    End Sub
End Module