Touch screens are very convenient because one can directly point to where it is interesting. This paper presents an inexpensive technique to transform an ordinary screen into a touch screen using in ordinary camera. The setup is easy: position a camera so it can see the whole screen. The system calibration involves the detection of the screen region in the image, which determines the projective mapping between the image plane and the screen. In order to compensate the non-flatness of the screen, an optional step can be applied by displaying a set of dots on the screen. In runtime, our system locates the indicator finger tip in our current implementation) in the image and converts the image position to the cursor position on the screen. Finger gesture or key input executes appropriate actions. The system has been demonstrated to be quite accurate, and its applications include kiosk and kids entertainment.