Toilet tank
How does it know the tank is full?
There's no chip in here, just a float on an arm. Flush, fill it back up, move the shut-off line, then hold the float up while the tank is still empty.
Fool it
Senses
Rule
Does
For grown-ups: what's really inside
A toilet's fill valve has a float, either a ball on an arm or a cup that slides up a tube. Water lifts the float, and once it rises far enough the arm presses a rubber seal shut. No electricity and no chip: the lever is the sensor, the rule and the switch all at once.
A single artificial neuron does the same job in numbers: take one input, compare it with a line, switch. Simplified: real valves close a little at a time as the float rises, and the flapper that empties the tank is a separate part.