That depends!
There are infrared touch screens that are essentially a frame that fires a grid of infrared lasers back and forth across the screen. When you touch the screen you deform the plastic and it cuts the beam. The X and Y laser you cut tells the software roughly where you touched.
Modern touch screens work on a similar principle only instead your finger is connecting a circuit across points on the glass and depending on which points are activated it can tell where you are touching.
The modern way has several advantages. First, you can multi-touch without one spot interfering with another. Second, you can infer pressure by how wide an area they are triggering. And third, the structures they are using are very small and that allows for significantly more precise touch target selection.
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