The code in the example is using `analogRead()`

, which reads the voltage on the `P0`

nail and compares it to the 3 volts from the `3V`

nail.

Here’s a way to think about it: The `3V`

(they use `P1`

at the end) “puts” three volts into the soil. The `P0`

measures how much of that 3 volts was “lost” as it travelled through the soil to the `P0`

nail. It is doing a comparison (somewhat like subtraction) to see how much it’s changed…it needs both the “start” (`3V`

) and end (`P0`

) to do the comparison needed here.

If you haven’t studied voltage and current before, you might take a look at some tutorials. I like SparkFun’s: Voltage, Current, Resistance, and Ohm’s Law, which has a few videos that may help.

Bill