Now think.
If I push very hard into a large pipe,
then close the tap halfway — what happens?
Two things: one BEFORE the tap, one AFTER the tap.
BEFORE
To keep the same flow, pressure must build.
It is “harder” to press.
AFTER
Water can form a “jet” through the narrow gap,
but the pressure after the tap is lower. Why?
Why?
The tap restricts how much water flows. Most of the pressure drop is across the tap.
After the tap, the wide, unrestricted return pipe keeps the pressure close to the return pressure — even while water is flowing.
In our circuit, A0 is wired directly to GND, so it is at 0 volts relative to GND.
Light changes the LDR’s resistance and the current, but the reading stays near 0.