Voltage, current, and resistance, as water pressure, flow, and a narrow, choked pipe
← Narrow: more resistanceWide: less resistance →
Current: 0.0 mA
I = (V − Vf) ÷ R, same formula as Electronics 101
How the analogy maps
Voltage is water pressure: the height difference between the high reservoir (3.3V) and the low one (GND, 0V). No height difference, no push.
The GPIO pin is a valve. Setting it HIGH in your code opens the valve and connects the pipe to the high reservoir's pressure. Setting it LOW closes it again. No pressure gets through either way until it's open.
The LED is a one-way turbine. Water only spins it in one direction, and it needs enough pressure (forward voltage) before it starts turning and lighting up at all.
The resistor is a narrow pipe choked with pebbles. It doesn't care which direction, it just limits how much water can squeeze past no matter how much pressure is pushing.
More current isn't faster water, it's more of it. A narrower, more choked pipe doesn't speed the flow up or slow it down, it lets less through at all, same as fewer electrons passing by each second rather than the same electrons moving faster. Watch the dots and the LED's brightness, not their speed, as you move the resistor slider.
Real valves, turbines, and pebble-choked pipes don't look exactly like this, but the relationships hold: pressure pushes, a valve decides whether it can push at all, and a narrow enough passage limits how much gets through regardless of pressure.