It all began with the swing-arm airspeed indicator. A vane was suspended from the lower surface of the upper plane, with a calibrated scale on a flat panel immediately outboard of the vane arm. As airspeed increased, the angular displacement of the vane arm increased and the speed could be read off against the calibrated scale. Once pilots could tell how fast they were going (or not going, as the case may be) they wanted to know more, like how high they were, how much fuel was in the tank, what time was it and such like. Eventually we ended up with the modern cockpit, where a pilot has so much information available to him that he no longer has any idea what is going on, and leaves it all to the computers.
Don't complicate matters until you've explained the different methods used by Smith's and Kelvin and Hughes for temperature compensation of the capsule stack in their altimeters.