4B pencil, paper, alligator leads, volt meter = position sensor

voltage, resistance, current: a bucket brigade to transfer energy

into a digital input. more like 5V, or more like 0V?

a variety of switches, pull up resistors

analog input from the position sensor

analog input from a piece of conductive foam with a pullup resistor

analog input from a commercial force-sensitive resistor

analog light sensor

analog proximity sensor

digital proximity sensor

fancy sensors: motion sensors

[make a water analogy with a pump, a polyester fluff filled resistor, with four or five open top manometers along the length of the resistor.]