The Spring Inside

Your phone knows which way is up because it contains an accelerometer — a chip with a microscopic mass suspended on tiny silicon springs. Tilt your phone and gravity pulls that mass to one side, changing the capacitance between microscopic plates etched into the chip.

The phone reads that shift and works out exactly how you are holding it. The whole mechanism fits on a chip smaller than 3 mm across — too small to see with the naked eye.