Big Mess o’ Wires has a fun write-up of a project that these days is sort of a prime build for beginning makers: turning fruit and vegetables into controller keys! (Momentary buttons, basically.) You’ve likely seen variations of this idea, most notably by JoyLabz’s MaKey MaKey kit. This approach is basically the same – with an Arduino Uno as the brain – and the code provided is easily understood and modifiable. And the whole setup is clearly explained.
If the hand isn’t touching the fruit, then the whole fruit-hand-body section becomes an open circuit with infinite resistance. In this case, the circuit simplifies to just +5V connected through a 1 meg resistor to the analog input. Because the analog input draws virtually zero current by itself, there will be no current flowing in the circuit and no voltage drop across the 1 meg resistor (remember Ohm’s law V = iR, so when i = 0 then V = 0). The voltage measured at the analog input will still be +5V, and Arduino’s analogRead(A0) function will return 1023, the maximum possible value for its 10-bit resolution.
When the hand touches the fruit, the fruit-hand-body section forms an organic resistor of about 1 megaohm. Current will flow from +5V through the real 1 megaohm resistor, then through the fruit-hand-body 1 megaohm resistor and down to ground. The total resistance between +5V and GND is 2 megaohms, and with two equal value resistors, the voltage at the point midway between them will be half the total voltage drop. That means the Arduino’s analog input will see 2.5V, and the analogRead(A0) function will return a value around 512.