How does a touchscreen know where your finger is?

Your phone screen does not feel pressure. It feels electricity — yours.

Under the glass is an invisible grid of thin wires that holds a small electric charge. The human body conducts electricity, so when your fingertip comes close, it pulls a tiny amount of charge from the grid at that exact spot. The phone notices where the charge changed and knows where your finger is, dozens of times a second.

This is why a plastic pen or a gloved finger does nothing: plastic and wool do not conduct. It is also why a wet screen goes wild — water conducts too, and the phone sees fingers everywhere.

Older touchscreens, like those on railway ticket machines, worked by pressure: two layers pressed together. They worked with anything, but they could not do multi-touch, so pinch-to-zoom had to wait for the electric kind.