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.

Most modern smartphones use capacitive touchscreens. These touchscreens are made of glass. 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.