How does a touchscreen work?

There are several different types of touchscreens. The two that you're probably most familiar with are resistive and capacitive.

Resistive touchscreens, which are used in Nintendo's products and pre-iPhone PDAs and smartphones have flexible plastic screens. When you push on the screen, you squeeze multiple layers together and this completes an electric circuit.

The most common form of touchscreens these days is "capacitive" touchscreens. These touchscreens are made of glass. Most modern smartphones use capacitive touchscreens. What does capacitive mean? That they use capacitors! Now capacitors are this weird thing where you can store electricity in two things that are close but not touching.

How does a touchscreen work?

The classical example is two metal plates separated by air. It turns out that the electric field between them can store energy, and the closer they are together, the more energy they store. When you touch the screen with your hand, you distort the electric field in the screen and it can measure where that change took place. Insulators, like plastic or most fibers, won't distort the field so the screen won't recognize them. "Smartphone gloves" have metal fibers woven into the fingertips to make the screen notice them.

The other type is infrared. It's less accurate than the other two types but cheaper for large-scale installations (museums, airports, etc.).

These work by shooting a beam of infrared light across a surface, typically using LEDs and/or lasers. A camera or set of cameras watches the surface for your finger interrupting the beam of light and interprets that as a touch.

Before capacitive touchscreens became widespread infrared was also favored for durability where the flexible membrane needed for a resistive screen wouldn't quite hold up. They're still used occasionally in industry because it's the only durable touchscreen technology that can be used with gloved hands.