Sub Form_Click ()
    Dim Msg, Number                        ' Declare variables.
    Msg = "Enter a non-negative number."
    Number = InputBox(Msg)                 ' Get user input.

    If Number < 0 Then
        Msg = "Cannot determine the square root of a negative number."
    Else
        Msg = "The square root of " & Number & " is "
        Msg = Msg & Sqr(Number) & "."
    End If

    MsgBox Msg                             ' Display results.
End Sub