There are ways and there are ways of building a touchscreen, but almost all of them imply using a special layer that attaches over the surface of the original display and interprets your touch points into coordinates that are then transmitted to a computer, which applies your action.

    One user tried to do it differently, using range sensors mounted on the side of the display, so that they pick up the distance from them to your hand, thus computing where exactly on the screen you are pointing. Below is a short video demo on how all this DIY touchscreen using range sensors works.